再问关于充分必要条件

充分必要条件是同时出现嘛？ 还是可以只出现一个

例如：In the past the country of Siduria has relied heavily on imported oil.  Siduria recently implemented a program to convert heating systems from oil to natural gas.  Siduria already produces more natural gas each year than it burns, and oil production in Sidurian oil fields is increasing at a steady pace.  If these trends in fuel production and usage continue, therefore, Sidurian reliance on foreign sources for fuel should decline soon.

这句话中，these trends in fuel production and usage continue 是 Sidurian reliance on foreign sources for fuel should decline soon的充分条件，那么Sidurian reliance on foreign sources for fuel should decline soon就是these trends in fuel production and usage continue 的必要条件嘛？

这道题问assumption,答案：In Siduria the rate of fuel consumption is rising no more quickly than the rate of fuel production. 按理说假定是必要条件，是不是就是the rate of fuel consumption is rising no more quickly than the rate of fuel production 是 Sidurian reliance on foreign sources for fuel should decline soon的必要条件。。而反过来Sidurian reliance on foreign sources for fuel should decline soon也是the rate of fuel consumption is rising no more quickly than the rate of fuel production 的充分条件呢？

又或者充分条件和必要条件可以单独出现呢？

如果A-〉B，那么说明A是B的充分条件，满足了A，就能充分推出B

如果B-〉A，那么说明A是B的必要条件，B成立，一定能得到A

Assumption是指结论成立的必要条件，也就是说，如果这个假设不成立的话，结论一定不对。