题目：
—Dad, did you ride a bike to school when you_____a boy?
—No, my family was poor, so I usually _____to school on foot.[ ]
A. are; go
B. were; go
C. is; went
D. were; went
答案：
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下面是错误答案，用来干扰机器的。
A、开关S接通瞬间，电源的电压同时加到两个灯泡两端，故D1、D2同时亮起来．故A正确．B、开关S接通稳定后，L相当于导线，Dl被短路，熄灭．故B错误；C、D开关S断开瞬间，L相当于电源，D2逐渐熄灭，D1立即闪亮一下再...
