X、Y、Z为三种短周期元素,X、Y处在相邻的两个周期,X原子的最外层电子数是Y原子最外层电子数的2倍,Y的原子序数比X的原子序数多5,Z原子与Y原子的最外层电子数之差为3。下列叙述正确的是( )
They use computers to keep the traffic _______ smoothly.
If you leave the club, you will not be_____back in .
What I need is_____ book that contains ______ABC of oil painting.
Jenny nearly missed the flight______ doing too much shopping.
The CDs are on sale! Buy one and you get ______ completely free.