前牙PFM作龈下边缘的主要优点有（）。
A.美观
B.不刺激牙龈
C.有利于自洁
D.防止菌斑附着
E.修复体边缘密合性好
参考答案：A
简述不落家形成的原因.
I want to improve my learning efficiency(效率) _____ have more spare time for my hobbies.[ ]
A. because of
B. in order to
C. as a result
D. so that
