题干
_____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
答案(点此获取答案解析）
D
_____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
”和“
”分别表示质子数不同的两种原子，其中表示化合物的是（ ）