问题描述：
By the time the fire engines arrived,the house__to the ground.
a was burned
b has been burned
c had been burned
d should have been burned
by the time是用于对将来的假设还是什么呢?
a was burned
b has been burned
c had been burned
d should have been burned
by the time是用于对将来的假设还是什么呢?
问题解答：
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