John's score on the test is the highest in the class ; （）he last night.
C
「句意」约翰的考试分数全班最高;他昨天晚上一定学习了。
「难点」表示对过去某一动作行为的猜测须要用musthavedone这一句型结构。
_____ if I had arrived yesterday without letting you know beforehand.
It is not uncommon for there _____ problems of communication between the old and the young.
Thebackgardenofourhousecontainsalawn，（）verypleasanttositoninsummer.