电梯安装、改造和维修单位承担电梯的日常维护保养，应当达到国家（）标准的要求。
A.一般性
B.强制性
C.非强制性
D.试验
参考答案：B
—He worked out such difficult questions at a very young age.
—Oh, he _______a very smart student then [ ]
A. could have been
B. should have been
C. might have been
D. must have been
心肌梗死24小时内并发急性左心衰竭时，最不宜应用（）。
A．吗啡
B．洋地黄
C．利尿剂
D．硝酸甘油
E．多巴酚丁胺
