Question

【题目】（2017·浙江温州· 6）________ the scientists have done lots of research on Mars，there is still much waiting to be discovered.

A. If B. Since C. Unless D. Though

Answer 1

【答案】D

【解析】句意：虽然科学家已经对火星做了大量研究，但仍然有很多未解之谜等待揭晓。A. If如果； B. Since自从；C. Unless除非；D. Though虽然。根据题意，可知前后句关系是“虽然…但是…”，though/although不能和but同时出现，故答案为D。