A:△EFB
B:△DEF
C:△CFB
D:△EFB与△DEF
I went to school without ____ (eat) anything this morning.
太阳光在真空中的传播速度是____m/s.太阳光通过三棱镜后被分解成七种颜色的光的现象叫做光的____.
从古猿进化到人,具有决定意义的一步是( )