An echo is an example of MCQ with Answer and Explanation

Explanation:
Sound travels to cliff and back. Total distance = speed × time = 340 × 2 = 680 m. So distance to cliff = 680/2 = 340 m. Use formula d = (v × t)/2. Time for round trip. Minimum distance for echo 17 m (approx) for 0.1 s persistence.

Explanation:
Cadmium or boron absorb neutrons to control chain reaction.

Explanation:
Outside magnet, lines from North to South. Inside magnet, they go from South to North, forming closed loops. Magnetic field lines continuous, no start/end.