病毒蚀斑技术是指病毒在已长成的单层细胞上形成的下列哪一种改变（）
A.细胞圆缩
B.细胞死亡
C.局限性病灶
D.细胞融合
E.细胞脱壁
参考答案：C
I want ____________ a map of China.[ ]
A. buy
B. is buying
C. to buy
If you want to learn English well, you must________ English as often as possible.[ ]
A. practice to speak
B. practice speaking
C. practicing speaking
D. to practice speak
