A:4
B:﹣4
C:3
D:﹣3
改错
What's this? It's an pen. ____
如图在一块玻璃砖内,一束平行光线恰好正对着玻璃内铁饼形状的空气泡射去,则光束通过空气泡( )
64-6____64-60+6 75-72____75-27+8
20+8+13____20+(8+13) 96-70____16+70