题目

_____ to manage time wisely, and you can make the most out of each day. A . Learning B . To learn C . Learned D . Learn 答案：D铁锤质量为M，从高H的地方自由下落，打击质量为m的木桩，木桩受土层的平均阻力为F，每次打击后铁锤与木桩一起下落.每次能把木桩打入的深度为____________.