升降机上的传动系统应设有()制动器。
A、长开式
B、长闭式
C、手动式
D、3种都可以
参考答案：B
My brother is an actor. He ______ in several films so far.[ ]
A. appears
B. appeared
C. has appeared
D. is appearing
燃烧2mol某有机物，可以收集到标准状况下89.6L的CO2和6mol的H2O，试推断该有机物的分子式可能是（ ）
A．C2H4
B．C2H6O
C．C2H6
D．C3H6O
