两种元素原子的核外电子层数之比与最外层电子数之比相等，则在核电荷数为1～18的元素中，满足上述关系的元素共有（   ）

A．3对

B．4对

C．6对

D．5对

A certain Social Security recipient will receive an annual benefit of $12,000 provided he has annual earnings of $9,360 or less, but the benefit will be reduced by $1 for every $3 of annual earnings over $9,360. What amount of total annual earnings would result in a 50 percent reduction in the recipient’s annual Social Security benefit ？（）(Assume Social Security benefits are not counted as part of annual earnings.)

A．$15,360

B．$17,360

C．$18,000

D．$21,360

E．$27,360