P = power of transmitter

Intensity at reflector = I1 = P/4p R2

Power reflected = P1 = P= I1A1, where A1 is the area of the reflector.

P1 = PA1/4p R2

The intensity at the receiver (same distance) = I2 = P1/4p R2 = PA1/16p 2R4

If A2 is the area of the receiver, then the power detected = I2A2 = PA1A2/16p 2R4 = 1X10-10P

R4 = A1A2/16p 210-10 = (p /4)(0.5)2(1)/(16p 210-10) = 1.243X107

R = 60 m

Answer is B

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