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答案：A
解析：
First you can change 1 minute to 60 seconds so that the ratios are both in envelopes per second. One inserting machine inserts letters at the rate of 120 per 60 seconds, or 2 per second. So 18 machines would insert 36 letters per second.
Let x be the number of stamping machines needed to keep up with 18 inserting machines. Then, since one machine stamps 3 envelopes per second, x machines stamp 3 times x envelopes per second. You can write the equation 3 times x = 36, which gives x = 12.
