The missing boys were last seen _____ near the river.

A．playing

B．to be playing

C．play

D．to play

已知在直角ABC中，∠C=90°，AC=8㎝，BC=6㎝，则△ABC的外接圆半径长为（    ）㎝，△ABC的内切圆半径长为（    ）㎝，△ABC的外心与内心之间的距离为（    ）㎝。