Echo time difference determines sound speed

Steps:

• Round-trip distance to P: 2 × 50 m = 100 m
• Round-trip distance to Q: 2 × 200 m = 400 m
• Time for Q echo = time for P echo + 1 s, so 1 s = (400 m - 100 m) / v
• v = 300 m / 1 s = 300 m/s

Why B is correct:

• Echo time equals round-trip distance divided by speed; 1 s difference matches 300 m extra distance at 300 m/s.

Why the others are wrong:

• A: Implies 1.2 s difference (300/250), not 1 s
• C: Implies 0.857 s difference (300/350), too short
• D: Implies 0.6 s difference (300/500), too short

Final answer: B