题干
—How _____ you have been late then in such a small town?
—Lots of turns in the streets, and the buildings look all the same.
—Lots of turns in the streets, and the buildings look all the same.
A：ca
B：could
C：may
D：must
答案(点此获取答案解析）
B
和 
分别表示两种不同元素的原子，那么其中表示混合物（ ）