题目

I _____ you somewhere before , but your name has escaped me _____ moment .         A. must see ; for a moment                 B. must have seen ; for a moment         C. must see ; for the moment                D. must have seen ; for the moment 答案：D 解析: must与不定式的完成体连用表示对过去发生的事持肯定态度的推断，在疑问句中用can，在持否定态度的推断时用can not 。例如：He must have rid himself of his preumonia last week , for you see , he doesn't cough at all now . (他在上星期肯定治愈了他的肺炎，你看，他现在一点都不咳嗽了。) for a moment表示一个动作延续如图所示的电路中电源电压为6V，开关S断开时电流表的示数为0.3A，开关S闭合时电流表的示数为0.5A，则R1的阻值为________Ω，R2的阻值为________Ω，R1消耗的功率是________W，2s内整个电路消耗的电能是________J．