A Physicist Wishes to Measure the Exposure of a Beam

Multiple Choice

A physicist wishes to measure the exposure of a beam of gamma rays. The beam is passed through 2.00 × 10^-2 kg of dry air at STP. The beam produces positive ions in the air which have a total charge of 3.87 × 10^-6 C. What is the exposure (in roentgens) of the beam?

A) 7.74 × 10^-8 R
B) 1.94 × 10^-4 R
C) 3.25 × 10^-2 R
D) 0.750 R
E) 1.25 R