一氧化碳浓度达到（）ppm可以工作15～20分钟。
A、40
B、80
C、160
D、24
参考答案：C
— You must have seen the great waterfall there.
— Sure. It is really a wonder, _____ that still seems right in front of me. [ ]
A. one
B. it
C. that
D. which
二进制数101001转换成十进制整数等于( )。
A．41
B．43
C．45
D．39
