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The book was so difficult that he read it for three hours___

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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?
这句话表示过去的事实.(与现在没有任何联系,不能用现在完成时)
意思是:这本书很难,他读了三遍直到读懂.
read 是过去式与understood 时态一致