合成机构是滚齿机（）传动中不可缺少的机构。
A.主运动
B.分度运动
C.进给运动
D.差动运动
E.切入运动
参考答案：D
The problem in today's parenting environment is ________ many believe it is their job to solve all their
children's problems. [ ]
A. who
B. that
C. which
D. what
婴幼儿毛细血管采血法的采血部位应选择何处为宜（）
A.耳垂
B.指尖
C.脚趾
D.足跟
E.头皮
