单项选择题


A．but
B．then
C．and
D．however
<上一题 目录 下一题>
热门 试题

单项选择题
In his research at Columbia University, Charles Townes worked with all of the following EXCEPT _________.
A．stimulated emission
B．microwaves
C．light amplification
D．a maser
单项选择题
In approximately what year was the first maser built
A．1917.
B．1951.
C．1953.
D．1957.
相关试题