工件表面形状不同时耦合效果不一样，下面的说法中，哪点是正确的（）
A.平面效果最好
B.凹曲面居中
C.凸曲面效果最差
D.以上全部
参考答案：A
根据句首字母补全单词。
1. I have to make a p_____for my study.
2. School is closed for the summer and children are on v_____.
3. New year is coming. I am going to post a p_____to my friend.
4. Parents must s_____their children to school.
5. He is a_____from home for a long time.
“马→蛔虫→原生动物”属于（）类型食物链。
A.捕食
B.腐食
C.混合
D.寄生
