The substance used as moderator in nuclear reactor must 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:
The number of half-lives (n) that have passed is total time / half-life = 15 / 5 = 3 half-lives. The fraction of the sample remaining undecayed is given strictly by the formula (1/2)^n. Therefore, the fraction remaining is (1/2)^3 = 1/2 * 1/2 * 1/2 = 1/8.

Explanation:
The equation of trajectory represents the path of the particle in the x-y plane, eliminating the time variable 't'. Given x = 2t, we can write t = x/2. Substitute this into the y equation: y = t^2 = (x/2)^2 = x^2 / 4. This confirms the particle moves in a parabolic path.