That was the first time I _______ there and I was impressed by the friendly people very much.

A．go

B．was going

C．had gone

D．have gone

下列对空气的描述不正确的是[ ]

A．按质量分数计，空气中含氧气约21%，含氮气约78%

B．空气的成分是比较固定的

C．空气的成分以氮气和氧气为主

D．空气不是一种单一的物质，而是由多种气体组成的混合物