---Mr Smith spent as much time as he ________ the experiments．---No doubt he succeeded in the end．

---Mr Smith spent as much time as he ________ the experiments．

---No doubt he succeeded in the end．

A.could do

B.could to do

C.could doing

D.could on doing

答案及解析： C

解析：

本题涉及到spend time(in)doing sth．和as much…as one can结构．