外国人前往不对外国人开放的地区旅行,必须向当地旅游行政管理部门申请旅行证件,由旅游者临时居留地或工作地的市、县旅游行政管理部门办理。( )
参考答案:错
Jack wasn't saying anything,but the teacher smiled at him ___ he had done somethingvery clever. [ ]
A. as if
B. in case
C. while
D. though
重介旋流中物料所受()影响较大。
A.阻力
B.重力
C.浮力
D.离心力