单选题 A series of attempts ______made, he came to a successful solution of the problem.
A. to be B. had been
C. were D. having been

【正确答案】 D
【答案解析】[解析] 考查考生对时态的判断，因为那些试验已经过去了，要用过去完成时，再加上在同一个句子里面不能出现两套主谓宾，所以得用从句的形式，因此选项D是正确答案。