在气阀机构中（）是工作条件最恶劣的零件。
A、气阀和阀座
B、阀杆和导管
C、内外气阀弹簧
D、卡环和撞击块
参考答案：A
They _______ themselves on the beach when the rain ________.[ ]
A. were enjoying; came
B. are enjoying; comes
C. enjoyed; is coming
D. were enjoying; was coming
股骨干骨折骨骼牵引适用于（）
A.儿童
B.较大的儿童
C.老年人
D.体弱的成年人
E.青壮年
