Pompeii _______ by so many tourists that some of the relics are not well protected.

A．has been visited

B．is visited

C．is visiting

D．was visited

关于氮气的特性描述正确的是（）

A.氮气是一种无色无味无嗅的气体

B.处在94%以上氮含量环境中会产生窒息

C.氮气的化学性质不活泼

D.氮气是氧化剂