绘图题：画出德士古煤炭气化工艺流程。
参考答案：
Helen and Mary are good friends. _______ often help each other. [ ]
A. Them
B. Theirs
C. They
D. Their
铁路合同包括______。
A．旅客车票
B．行李票
C．包裹票
D．站台票
