齿轮传动能保证两轮瞬时传动比恒定。( )
参考答案：对
4.02吨=______吨______千克；2时12分=______时；0.5米=______厘米；30平方分米=______平方米．
People now sleep about 20% less than they did a century ago. One 2010 poll of 1000 Americans found 36% are drowsy or fall asleep when they are driving, and 29% become very sleepy at work.
