题干
More than two thirds of the land of the expo garden ________ with trees and grass in the past few years.
A：is covered
B：are covered
C：has been covered
D：have been covered
答案(点此获取答案解析）
C
ABCD中，过点D作DE⊥AB于点E，点F 在边CD上，DF=BE，连接AF，BF．
,人们把这种波称为物质波,也叫德布罗意波.如果有两个电子的速度分别为
和
，且
.则这两个电子对应的德布罗意波的波长关系为
