活塞环的油环主要作用是布油和（）。
A.密封
B.导向
C.导热
D.刮油
参考答案：D
— Do you hear the noise outside?
— Yes. There must be some kids _____ outside. [ ]
A. have fun
B. has fun
C. to have fun
D. having fun
如何防止酱腌菜制作过程的褐变反应？
