商品混凝土拌合机按搅拌筒的形状分为锥式、盘式、（）、和槽式以及鼓筒式搅拌。
参考答案：梨式
_____several times，but unfortunately，the boy still couldn’t smooth away the problem on his own.
A．Having been explained
B．The problem having been explained
C．The problem had been explained
D．Although the problem had been explained
给排水设计中，管线综合的原则是什么？
