Sound cannot travel through: MCQ with Answer and Explanation

Explanation:
Echo is the reflection of sound waves from a rigid surface, returning to the listener after a time delay. For distinct echo, minimum distance to reflector is about 17 m (so time delay > 0.1 s, persistence of hearing). Refraction bends sound in media with varying density; interference combines waves; diffraction bends around obstacles. Memory tip: 'Echo = sound reflection like light in mirror'. This application-based question links wave properties to daily-life phenomena, frequently tested in competitive exams to assess conceptual clarity beyond definitions. Always distinguish echo from reverberation (multiple reflections).

Explanation:
Heavy water (D₂O) slows neutrons, good moderator.

Explanation:
Biot-Savart law: field forms concentric circles around wire, direction by right-hand thumb rule. Magnitude B = μ₀I/(2πr), decreasing with distance. Memory tip: 'Right-hand rule: thumb = current, fingers = field circles'. Fundamental magnetism concept frequently tested in competitive exams.