to reach them on the phone, we sent an email instead.
A. Fail
B. Failed
C. To fail
D. Having failed
网友回答
A. Fail
B. Failed
C. To fail
(答案→)D. Having failed
解析：句意：电话没有打通，我们就发了一个电子邮件。这里逗号后是句子，前面没有连词，故用非谓语动词，这里fail与we之间是主动关系,并且是先是打电话，然后发的电子邮件，故用分词的完成时，故选D。