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。
Thefarmingmethodshavebeen________improvedwiththeinventionofthenewmachine.
Sincewehaveafocusedsubject，weshouldnottalk________.
Thestudentsarerequiredto________themainideasofthearticleintheirownwords.
Thevehiclewastravelingatspeeds________90milesperhour.