同一件铜钟为什么能发出两种不同的乐音？
这两种乐音是由钟的侧部和中央部位发出的，人们称之为侧鼓音和正鼓音。
—May we go home mow?
—No, you _______. You _______ to go home until 5 o'clock.[ ]
A. mustn't; are allowed
B. don't have to; are supposed
C. needn't; aren't allowed
D. can't; aren't supposed
等到物质的量规则
