对防护草皮的要求不包括（）。
A.适应能力强
B.管理费用低
C.防冲效果好
D.进口美化草
参考答案：D
— Did you write to Grace last summer?
— No, but I ______ her over winter vacation. [ ]
A. will be seen
B. will be seeing
C. will have seen
D. will have been seen
调节器和调节对象相互作用所形成的闭合回路称为（）。
