Question

【题目】_____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进行了大量研究，但仍有很多有待于去发现。if如果；since自从；unless除非；though虽然。根据前后关系可知表示让步，故选D。