答案：A

解析：

If a question looks as if it requires a lot of calculating, that’s often a tip-off that something else is going on. There’s usually a quick way to find the answer. In this question, all the fractions are being multiplied and so canceling is a possibility. The denominators cancel diagonally with the numerators that follow.

The 2 from 1/2 cancels with the 2 from 2/3.

The 3 from 2/3 cancels with the 3 from 3/4.

And so on, right down to the equal sign.

After you have canceled everything that can be canceled, you are left with the fraction 1/7.