—When I first got to Harvard, my awkward English brought me great trouble.

——Oh, dear, it _______a struggle for you to keep up with other students.

A．might be

B．can be

C．could have been

D．must have been

义齿撬动或摆动的原因是

A．支托移位形成支点
B．基托边缘过长或过锐
C．基托过厚过大
D．卡环臂进入基牙倒凹区过深
E．咬合过高，基牙负担过重