Since we have a focused subject ，we should not talk ________.
D
译文：既然我们已经有了命题，我们就不应漫无边际地交谈。此题考查对固定短语的掌握情况。atonce立刻;athand在附近，在手头，即将发生;atintervals间或，不时;atrandom随便，任意。
Robots differ from automatic machines ________ after completion of one specific task，they can be reprogrammed by a computer to do another one.
A year later，with the nation’s economy ________，Bush’s approval rating dropped below 40 percent
Although Tom is satisfied with his academic achievement，he wonders _______ will happen to his family life .
Ayearlater，withthenation’seconomy________，Bush’sapprovalratingdroppedbelow40percent
The vehicle was traveling at speeds ________ 90 miles per hour.