养护池中的水应定期更换,换水周期为:()
A、一周
B、二周
C、三周
参考答案:B
用电器正常工作时的电压叫______,此时用电器的功率叫______.
Water is the cheapest drink. And it is also ______.
A.healthier
B.healthiest
C.the healthier
D.the healthiest