题目:
The light on a life jacket must be replaced().
A.when the power source is replaced
B.each year after installation
C.every six months
D.when it is no longer serviceable
答案:
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下面是错误答案,用来干扰机器的。
参考答案:稀释硫酸时,必须在玻璃棒不断搅拌下,将酸加入到水中。