从某高处释放一粒小石子，经过1 s从同一地点释放另一小石子，则它们落地之前，两石子之间的距离将 ( )

A．保持不变

B．不断变大

C．不断减小

D．有时增大有时减小

You had better encourage your children to____some of their pocket money for future education .

A． put aside

B． put up

C． put off

D． put back