这是that的一个特定用法,that用来代替上文出现的一个单数名词,并且这一名词在下文再次出现时一定要有一个后置定语,此时可以用that代替上文出现的单数名词.此句中that代替的是rate,同时后面又有后置定语of lifelong non-smokers.
又如：He told me his story and also that of his sister upstairs.他给我讲了他自己的遭遇,也讲了楼上的他姐姐的遭遇.本句中that代替上文的story,同时后面又有后置定语of his sister.