前后都有缺隙的孤立前磨牙上最好设计

A．单臂卡环

B．三臂卡环

C．对半卡环

D．圈形卡环

E．U形卡环

______ the window. It's clean. [ ]

A. Not clean

B. Aren't clean

C. Don't clean

D. Clean