系统各项系数的符号必须大于零且a0a3＜a1a2是( )阶系统稳定的必要条件。
A．1
B．2
C．3
D．4．
参考答案：C
He has _____ healthy food. [ ]
A. a lot
B. a lot of
C. lots of
D. B and C
A．F1
B．F2
C．F3
D．无法判断
