David doesn’t give up he has failed in the experiment four times.A. ifB. whileC. thoughD.

问题补充：

David doesn’t give up he has failed in the experiment four times.

A. if

B. while

C. though

D. since

答案：

A. if

B. while

(答案→)C. though

D. since

解析：连词辨析。if 是否，如果；while当……时，然而；though虽然，尽管；since自从……以来，由于。句意：尽管他在实验中已经失败了四次，但是戴维德还不放弃。故选C。