设R上的偶函数f(x) 满足f(x+2)+f(x)=0 ，且当0 ≤x ≤1 时，f(x)=x, 则f(7.5)= .
0.5
21. — Jay Chou is in our city. Let’s go and watch his show this Saturday.
— That’s great, but I need to call my dad and ask for _________ before I go out.
A．permission
B．protection
C．invitation
D．Explanation
预聚合反应器的夹套水采用（）撤热。
A、循环水
B、冷冻水
C、工艺水
D、冷凝液
