生产缠绕胶管的纤维缠绕常用的哪两种？
参考答案：
常用的盘式缠绕机和鼓式缠绕机。
His writing is so confusing that it's difficult to make out ________ it is he is trying to express.(2011·安徽，33)
A．that
B．how
C．who
D．what
活塞环具有自紧密封的特点。
