Sincewehaveafocusedsubject，weshouldnottalk________.
A、atonceD
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。 at once 立刻; at hand在附近，在手头，即将发生; at intervals 间或，不时; at random 随便，任意。
Robotsdifferfromautomaticmachines________aftercompletionofonespecifictask，theycanbereprogrammedbyacomputertodoanotherone.
The outline of rooftops and chimneys ________ against the pale sky.
The oceans do not so much divide the world _______ unite it
When_______,waterwillbeturnedintosteam.
Thatproposalisessentiallyirrelevant________theissueathand.
