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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