题目

They had to_________ before they finished the course because they were not well disciplined. A.drop in                           B.drop out C.drop off                      D.drop by 答案：B如图所示，OO′是半圆柱形玻璃体的对称面和纸面的交线，A、B是关于OO′轴等距且平行的两束不同单色细光束，从玻璃射出后相交于OO′下方的P点，由此可以得出的结论是（ ）A．在玻璃中，A光比B光的速度小B．玻璃对A光的折射率比对B光的折射率小C．空气中，A光的波长比B光的波长长D．A光的光子能量比B光的光子能量大