____ help the people in the snowstorm in
题型：选择题
问题：
____ help the people in the snowstorm in the south made him worried. [ ]
A. Not known how to
B. Unknown what to do to
C. His knowing not how to
D. His not knowing how to
题型：选择题
如图所示，△ABC为一直角三棱镜的截面，其顶角为θ=30°。P为垂直于直线BCD的光屏，现一宽度等于AB的单色平行光束垂直射向AB面，在屏P上形成一条宽度等于
的光带，试作出光路图并求棱镜的折射率。（其中AC的右方存在有折射率为在的透明介质）
