问题
单项选择题
What do you find difficult in learning English?（）
A.Listening and speaking.
B.Don‘t be afraid of difficulties.
C.Learning English is useful.
D.Don‘t be disappointed.
答案
参考答案：A
搜题请搜索小程序“爱下问搜题”
What do you find difficult in learning English?（）
A.Listening and speaking.
B.Don‘t be afraid of difficulties.
C.Learning English is useful.
D.Don‘t be disappointed.
参考答案：A
的透明球表面，入射角为45°，在球的内壁经过一次反射后，从球面射出的光线为b，则入射光线a与出射光线b之间的夹角α为( )