Read the following SAT test question and then click on a button to select your answer.

A machine can insert letters in envelopes at the rate of 120 per minute. Another machine can stamp the envelopes at the rate of 3 per second. How many such stamping machines are needed to keep up with 18 inserting machines of this kind?

(A)12

(B)16

(C)20

(D)22

(E)24

来源:可可英语 http://www.kekenet.com/sat/201404/284601.shtml