At 1.0 m: L = 80.0 = 10 log(I1/I0)

I1/I0 = 108

I1 = 108I0 (when I0 = 10-12 W/m2)

Inverse square law: I = P/4 r2

I1/I2 = (r2/r1)2

I2 = (I0X108)(1.0/20.0)2 = I0X108X20-2

IL2 = 10 log(I2/I0) = 10log [(I0X108X20-2)/I0] = 80 - 26 = 54 dB

Answer is C

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