斜方肌上部和胸锁乳突肌可以使肩胛骨（）
A.上提
B.下降
C.前伸
D.后缩
E.上回旋
参考答案：A
Mike is said Chinese in a Chinese university. Do you know where he studied? [ ]
A. to have studied
B. to study
C. to be studying
D. to have been studying
如图所示，一束激光从O 点由空气射入厚度均匀的介质，经下表面反射后，从上面的 A 点射出；已知入射角为 θ 1 ，A 与 O 相距 l ，介质的折射率为 n ，光在空气中的传播速度为 c ．试求光在介质中的传播时间 t ．
