Microwaves and light both travel at speed c

Steps:

• Distance between satellites is 100 km.
• Microwave pulse from X to Y takes time T, so speed of microwave is distance/time = 100 km / T.
• Microwaves are electromagnetic waves, traveling at speed of light c in vacuum.
• Light pulse from Y to X covers same 100 km at speed c, so time = distance/c = T.

Why D is correct:

• Time = distance/speed; same distance and speed c for both pulses means same time T (electromagnetic wave propagation law).

Why the others are wrong:

• A: 10^T implies exponential growth unrelated to distance or speeds.
• B: 10^{-T} suggests inverse exponential, not matching wave speeds.
• C: Duplicate of A, no physical basis.

Final answer: D