抗血清长久保存的最佳条件是（）。 A.0℃ B.4℃ C.真空干燥 D.－20℃ E

At the time of the earthquake Jeff was still in his office. He ________ to finish a project before he left for the day.

A．has tried

B．had tried

C．has been trying

D．had been trying

________ favorite sport is soccer.[ ]

A. They                      

B. Them          

C. Theirs                     

D. Their

天使是否出现于各民族之间呢？

在轴承中，推力滑动轴承承受()载荷

A、径向及轴向

B、径向

C、随力的方向变化

D、轴向

EEBD要想供应使用者不少于10分钟的使用时间，储气瓶的储气量大于（）升.

A.200

B.300

C.400