对维持细胞内代谢恒定起着至关重要的酶调节方式是（）
A.反馈调节
B.同工酶
C.变构调节
D.共价修饰
E.多酶复合体系
参考答案：C
In the last few years, they ______ three major concerts. [ ]
A. have had
B. had had
C. had
