问题补充：
So fast that I could not catch up with them.
A. did they run
B. they had run
C. they ran
D. had they run
答案：
(答案→)A. did they run
B. they had run
C. they ran
D. had they run
解析：考查倒装句：在so... that…结构中，若将so+adj. / adv. 置于句首，则其后的主句要用部分倒装。句意：他们跑得如此快，以至于我不能赶上他们。由句意可知是过去时，提前did，选A。
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