I set up email addresses for connecting with my parents last May. But I didn’t write to th

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A. should
B. must have
(答案→)C. should have
D. must
解析：考查情态动词的完成式：句意：去年五月我给父母发了一份电子邮件想和他们联系，但是我没有象应该的那样经常给他们写信。因为讲的是过去的事情，所以是情态动词的完成式，排除AD项，must have done过去一定做过…，should have done本应该做而没有。选C。