题目

The group of students went to the suburb, did some research work, and ________ the water in the river to see if it had been polluted.A. proved          B. boiled           C. tested          D. exchanged 答案：C 解析：test表示“测试；测验”，以确认是否合格。prove意为“证明”；boil意为“煮沸”；exchange意为“交流；交换”。下列有关平衡力的说法，正确的是 ( )A. 物体在平衡力的作用下，一定是静止的B. 平衡力一定是作用在两个物体上的力C. 静止在桌面上的苹果受到平衡力的作用D. 处于平衡状态的物体，一定是受到两个平衡的力