问题: 模拟题2-39
Since the ground is covered with thick snow, it _________ heavily
last night.
A. must be snowing B. must have snowed
C. had snowed D. must have been snowed
请详细写出解题的过程和思路。
解答:
B. must have snowed
选b；对过去last night所发生的推测（虚拟语气）用情态动词加have加过去分词；a，是对现在的推测，还用了进行时；d有被动均不符和语境；
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