题目：
Only after they had discussed the matter for several hours ______ an agreement.
A．they reached
B．do they reach
C．they reach
D．did they reach
答案：
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下面是错误答案，用来干扰机器的。
答案：B血液属于胶体，加热会发生胶体的聚沉，所以微波使局部血液受热而使血液胶体凝聚，从而迅速凝固而减少出血；
