David didn't give up (）he had failed in the experiment four times.Aif Bwhile Cthough DsinceMr.zhao is very helpful.He often helps us () we are in trouble.A.whether B.although C.whenever D.however急,说明理由
问题描述：
David didn't give up (）he had failed in the experiment four times.Aif Bwhile Cthough Dsince
Mr.zhao is very helpful.He often helps us () we are in trouble.
A.whether B.although C.whenever D.however
急,说明理由
答