问题
选择题
—What's this?
—________. [ ]
A. It's green
B. It's a ruler
C. I'm fine
D. It's pen
答案
答案：B
搜题请搜索小程序“爱下问搜题”
—What's this?
—________. [ ]
A. It's green
B. It's a ruler
C. I'm fine
D. It's pen
答案：B
缺失，基牙
上设计正型卡环，
上设计杆式卡环，大连接体设计为舌板。