Two Boys Are Whispering in the Library

Multiple Choice

Two boys are whispering in the library. The radiated sound power from one boy's mouth is 1.2 × 10^-9 W; and it spreads out uniformly in all directions. What is the minimum distance the boys must be away from the librarian so that she will not be able to hear them? The threshold of hearing for the librarian is 1.00 × 10^-12 W/m².

A) 100 m
B) 35 m
C) 23 m
D) 16 m
E) 9.8 m