Recently our community has launched a campaign among all the residents , ____ we are called upon to do our little but to prevent the worsening environment.

A．where

B．which

C．as

D．when

设某循环队列的容量为50，头指针front=5(指向队头元素的前一位置)，尾指针rear=29(指向队尾元素)，则该循环队列中共有______个元素。