DNA复制过程中改变DNA超螺旋状态、理顺DNA链的酶是（）。
A.DnaA
B.DnaB
C.DnaC
D.DnaG
E.拓扑异构酶
参考答案：E
As is natural, a man as old as he ______ be very forgetful.
A．can
B．must
C．should
D．would
铁路暴风雨雪雾等恶劣天气应急预案》规定，铁路遇雨雪天气，客运部门应确保（）等处不得有影响旅客通行的积雪、积冰。
A、踏步台阶
B、以上都是
C、非露天站台面
D、地道、进出站口
As is natural, a man as old as he ______ be very forgetful.