diff --git a/01-url-health-checker/main.go b/01-url-health-checker/main.go
index 1302ba6..8928126 100644
--- a/01-url-health-checker/main.go
+++ b/01-url-health-checker/main.go
@@ -19,6 +19,10 @@ package main
 import (
 	"context"
 	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"sync"
 	"time"
 )
 
@@ -33,20 +37,138 @@ type Result struct {
 // TODO: implement me.
 func CheckAll(ctx context.Context, urls []string, limit int) []Result {
 	results := make([]Result, len(urls))
-	// TODO:
 	//   - semaphore (buffered channel) or worker pool for the concurrency limit
+	sem := make(chan struct{}, limit)
 	//   - sync.WaitGroup to wait for completion
+	var wg sync.WaitGroup
 	//   - http.NewRequestWithContext for cancellation
+	client := &http.Client{Timeout: 5 * time.Second}
+
+	for i, u := range urls {
+		wg.Add(1)
+		go func(i int, u string) {
+			defer wg.Done()
+			// wait for semaphore or ctx cancellation
+			select {
+			case sem <- struct{}{}:
+				defer func() { <-sem }()
+			case <-ctx.Done():
+				results[i] = Result{URL: u, Err: ctx.Err()}
+				return
+			}
+
+			start := time.Now()
+			req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+			if err != nil {
+				results[i] = Result{URL: u, Err: err}
+				return
+			}
+			resp, err := client.Do(req)
+			if err != nil {
+				results[i] = Result{URL: u, Err: err, Latency: time.Since(start)}
+				return
+			}
+			defer resp.Body.Close()
+			io.Copy(ioutil.Discard, resp.Body)
+			results[i] = Result{URL: u, Err: err, Status: resp.StatusCode, Latency: time.Since(start)}
+		}(i, u)
+	}
+
 	//   - write results[i] from goroutine i (why is this safe without a mutex?)
 	_ = ctx
 	_ = limit
+	wg.Wait()
 	return results
 }
 
 func main() {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
+	// Pick ONE of the url sets below (uncomment it, comment the others).
+	// Tip: run with a bigger `limit` and compare total wall-clock time.
+
+	// Option A — the original mix: OK, HTTP 500, DNS failure.
 	urls := []string{"https://example.com", "https://httpbin.org/status/500", "https://nope.invalid"}
+
+	//region Not-selected-options
+
+	// Option B — all succeed (2xx). Good for measuring how `limit` affects
+	// total time: with limit=1 it's the sum of latencies, with limit=8 ~max.
+	// urls := []string{
+	// 	"https://example.com",
+	// 	"https://www.google.com",
+	// 	"https://www.cloudflare.com",
+	// 	"https://github.com",
+	// 	"https://go.dev",
+	// 	"https://pkg.go.dev",
+	// 	"https://httpbin.org/status/200",
+	// 	"https://httpbin.org/get",
+	// }
+
+	// Option C — HTTP-level failures (connection succeeds, status is bad).
+	// Err should be nil for these; the caller decides what a "healthy" status is.
+	// urls := []string{
+	// 	"https://httpbin.org/status/200", // control
+	// 	"https://httpbin.org/status/301", // redirect — client follows it by default, so you'll see the final status
+	// 	"https://httpbin.org/status/401",
+	// 	"https://httpbin.org/status/403",
+	// 	"https://httpbin.org/status/404",
+	// 	"https://httpbin.org/status/429",
+	// 	"https://httpbin.org/status/500",
+	// 	"https://httpbin.org/status/502",
+	// 	"https://httpbin.org/status/503",
+	// 	"https://example.com/definitely-not-here", // real site, 404
+	// }
+
+	// Option D — network/transport failures (Err != nil, Status == 0).
+	// Each fails for a DIFFERENT reason — print r.Err and read the message.
+	// urls := []string{
+	// 	"https://example.com",                    // control
+	// 	"https://nope.invalid",                   // DNS: .invalid TLD never resolves
+	// 	"https://this-host-does-not-exist-12345.com", // DNS: NXDOMAIN
+	// 	"http://localhost:1",                     // TCP: connection refused (nothing listens on port 1)
+	// 	"http://10.255.255.1",                    // TCP: unroutable address → connect timeout (hits the 5s per-request timeout)
+	// 	"https://expired.badssl.com",             // TLS: expired certificate
+	// 	"https://self-signed.badssl.com",         // TLS: self-signed certificate
+	// 	"https://wrong.host.badssl.com",          // TLS: hostname mismatch
+	// 	"https://untrusted-root.badssl.com",      // TLS: unknown CA
+	// 	"ftp://example.com",                      // unsupported protocol scheme
+	// 	"not a url at all",                       // request creation fails (invalid URL)
+	// }
+
+	// Option E — slow responses: exercises the 5s per-request timeout AND the
+	// global ctx. /delay/N sleeps N seconds server-side before replying.
+	// With limit=2 and a 10s ctx, watch which ones finish, which time out
+	// per-request, and which never start and return ctx.Err().
+	// urls := []string{
+	// 	"https://httpbin.org/delay/1",  // OK
+	// 	"https://httpbin.org/delay/3",  // OK
+	// 	"https://httpbin.org/delay/8",  // exceeds 5s per-request timeout
+	// 	"https://httpbin.org/delay/8",  // exceeds 5s per-request timeout
+	// 	"https://httpbin.org/delay/1",  // may hit global ctx deadline before it even starts
+	// 	"https://httpbin.org/delay/1",
+	// 	"https://httpbin.org/delay/1",
+	// }
+	//endregion
+
+	// Option F — everything at once, larger list. Set limit to 1, 4, 16 and
+	// compare; check that the order of results still matches the input order.
+	urls = []string{
+		"https://example.com",
+		"https://go.dev",
+		"https://httpbin.org/status/200",
+		"https://httpbin.org/status/404",
+		"https://httpbin.org/status/500",
+		"https://httpbin.org/status/503",
+		"https://httpbin.org/delay/2",
+		"https://httpbin.org/delay/7",
+		"https://nope.invalid",
+		"http://localhost:1",
+		"https://expired.badssl.com",
+		"https://self-signed.badssl.com",
+		"https://www.cloudflare.com",
+		"https://github.com",
+	}
 	for _, r := range CheckAll(ctx, urls, 2) {
 		fmt.Printf("%-40s status=%d latency=%v err=%v\n", r.URL, r.Status, r.Latency, r.Err)
 	}