家庭电路中的保险丝是用电阻较大、熔点较低的铅锑合金制成的．当家庭电路发生短路时，保险丝会熔断．请分析说明保险丝熔断的原因．
用NA表示阿伏加德罗常数，下列说法正确的是( )
—Michael was late for Mr. Smith's chemistry class this morning.
—________? As far as I know, he never came late to class.
