单选题 Because of the great speed of light, we see a lamp light up almost at the exact moment ______.
A. we turn it on B. when we turn on it
C. that we turn on it D. which we turn it on

【正确答案】 A

【答案解析】[解析] turn on意为“开，旋开(电灯、自来水、无线电等)”，其中on是副词。“动词+副词”构成的动词短语后跟人称代词时，该人称代词必须放在动词和副词之间。例如：He suffers a great deal from smoking，so he has made up his mind to give it up．他深受吸烟之苦，因此决心戒烟。which作连接代词时，在其引导的名词性从句中充当某种成分。这样B、C和D均可排除，只有A为正确答案。