单选题 ______ a satisfactory answer, so he had to write to them again.
A. As he hadn't received B. Not having received
C. Having not received D. He hadn't received

【正确答案】 D
【答案解析】本题考查的是简单句与并列句的区别。根据so一词可以看出这是两个并列句，而不是一个简单句，所以前面也是完整的句子，故选D。因为……所以，在英语中这两个连词不能同时用，故不选A。