Students are required to go out in couples, especially in the evenings, with friends ______ they get into trouble．

A．even if

B．if only

C．in case

D．when

关于晶体的下列说法正确的是[    ]

A．在晶体中只要有阴离子就一定有阳离子

B．在晶体中只要有阳离子就一定有阴离子

C．原子晶体的熔点一定比金属晶体的高

D．分子晶体的熔点一定比金属晶体的低