X元素的阳离子与Y元素的阴离子具有相同的电子层结构,下列叙述正确的是
A.原子序数X<Y
B.原子半径X<Y
C.离子半径X>Y
D.原子最外层电子数X<Y
答案:D
两个平行平面间的距离为4,一条直线与两个平面所成角为45°,则这两条直线被两平行平面所截得的线段长为 .
The young _____ the vital forces in our society.[ ]
a. is
b. has been
c. are
d. have been