"Life expectancy" is the average age at death of the entire live-born population. In the middle of the nineteenth century, life expectancy in North America was 40 years, whereas now it is nearly 80 years. Thus, in those days, people must have been considered old at an age that we now consider the prime of life.
Which of the following, if true, undermines the argument above?
My summary from this question: 1. A unless B: A ->B (after UNLESS is a necessary condition) 2. To / in order to represents the if-part in if-then statements.
My summary from this question: 1. A unless B: A ->B (after UNLESS is a necessary condition) 2. To / in order to represents the if-part in if-then statements.
-- by 会员 AlohaDJ (2011/8/10 1:37:05)
A unless B: Not A --> B (You have to negate the claim in A to make it a sufficient condition).
Not A unless B: A --> B
-- by 会员 sdcar2010 (2011/8/10 2:35:13)
oh..YES!! Negation of A is indispensable here! THKS NN for pointing it out!!