Our goal is that you are completely satisfied with your purchase. If, for any
reason, you are not satisfied, simply return your purchase with your receipt f
or an exchange or refund within 90 days when ______ on your Sears Card.
a) purchase
b) purchasing
c) purchased
d) purchases
答案是C
我选的是b, 我自己是这样想的-when 后面的you 省略掉 所以purchase 加 ing。想请教
大家是我的想法哪里出错了吗？