人眼的“白眼球”和“黑眼球”实际上是指( )
A:虹膜和角膜
B:角膜和巩膜
C:角膜和虹膜
D:巩膜和虹膜
He said he would help me with my maths if he ____ free.
Breakfast is ready, you'd better____.
甲、乙两车同时同地反方向做匀速直线运动,它们的s﹣t图象如图所示.12秒钟时甲乙两车相距____ 米,以甲车为参照物,乙车是____ (选填“运动”或“静止”)的,甲车____ (选填“一定”或“可能”)受的是平衡力.