When I got to the bus stop. I missed the early bus and I had to____ the next one.
A:give up
B:keep off
C:call off
D:wait for
如果每盒钢笔有10支,总售价100元,那么购买钢笔的总钱数y(元)与所买支数x之间的关系式为____.
如图所示,空气中有一横截面为半圆环的均匀透明柱体,其内圆半径为r,外圆半径为R,R=r.现有一束单色光垂直于水平端面A射入透明柱体,只经过两次全反射就垂直于水平端面B射出.设透明柱体的折射率为n,光在透明柱体内传播的时间为t,若真空中的光速为c,则( )