Helen is on time for everything. How ______ it be that she was late for such an important interview?

A．might

B．can

C．must

D．may

有关氮气作用描述不正确的是（）。

A.防止油品氧化

B.防止或者减少油品挥发

C.防止设备氧化

D.正常生产时可以向常压精馏塔内注入少量氮气