If you are late for class,the teacher________ angry.[ ]
A. is
B. was
C. will be
D.will do
答案:C
如图所示,大口玻璃瓶有一些水,用打气筒给瓶内打气,当瓶塞弹起时,瓶内出现______,这说明______.如图所示,用细线将干净的玻璃板吊在弹簧测力计下面,记下弹簧测力计的示数.然后使玻璃板接触水面,稍稍用力向上拉玻璃板,发现弹簧测力计______(填“变大、变小或不变”),此现象说明______.
约束机制是指什么?