Some languages，________ Russian and German，are not easy to learn.

A．for example

B．such as

C．as

D．that is

位于我国地势第二级阶梯上的平原有[ ]

A．河套平原、成都平原

B．三江平原、宁夏平原

C．江汉平原、渭河平原

D．华北平原、鄱阳湖平原