取非意思就是假如某个选项是错的。
大体有三种情况:
1. all----not all/some---none/always----not always/sometimes----never/everywhere---not everywhere/somewhere---nowhere/only one, exactly one---none, more than one/at least one, some---none
2. 对于选项为条件句,将必要条件取非。
3. 直接将谓语肯定变否定,或否定变肯定。