正确答案是C.for（因为）的用法：
for是并列连词,表示推断的原因,或对前一分句补充说明理由.
例：It must be morning,for the birds are singing.
since（既然）语气较弱,强调已知的事实.since多用在句首,而且像D选项中这样后面加逗号绝对是错误的.
例：Since you can't answer the question,I'll ask someone else.
正确答案是C.for（因为）的用法：
for是并列连词,表示推断的原因,或对前一分句补充说明理由.
例：It must be morning,for the birds are singing.
since（既然）语气较弱,强调已知的事实.since多用在句首,而且像D选项中这样后面加逗号绝对是错误的.
例：Since you can't answer the question,I'll ask someone else.