矿井瓦斯比空气（）。
A、一样
B、轻
C、不确定
D、重
参考答案：B
下列不属于国家一级保护植物的是
A．桫椤
B．珙桐
C．水杉
D．地钱
I'm sorry to be late. Thank you for waiting.（）
A.Tell me the reason why you‘re late.
B.That‘s OK.
C.It really took up my much time.
D.It‘s a pleasure. I come here 10 minutes ago.
