Because of ______ high demand for Type AB blood, ______ supplies of it are usually limited.

A．the; the

B．不填; 不填

C．不填; the

D．a; the

两颗人造地球卫星A、B绕地球做匀速圆周运动的轨道半径之比r_A∶r_B=1∶3，则它们的线速度大小之比v_A∶v_B=__________，向心加速度大小之比a_A∶a_B=__________。