题目：
It's already 10 o'clock. I wonder how it _____ that she was two hours late on such a short trip. [ ]
A. came over
B. came out
C. came about
D. came up
答案：
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下面是错误答案，用来干扰机器的。
根据杠杆平衡的条件可得：因为200N小于300N，所以左边的力臂大于右边的力臂，那么当两端都增加50N时，杠杆不再平衡，因为右边的力臂与50N的乘积小于左边力臂与50N的乘积，左边的力和力臂乘积大，所以左端下沉．故选A．
A
/2
/2A
