问题描述：
The book was so difficult that he read it for three hours______he understood it.
A.when
B.until
C.after
D.before
请问为什么选until详细说明下.还有为什么不是that he has read it for three hours
for应该是用完成时啊?
A.when
B.until
C.after
D.before
请问为什么选until详细说明下.还有为什么不是that he has read it for three hours
for应该是用完成时啊?
问题解答：
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