一个钢质的圆柱体零件重1763.424克，它的侧面展开图是一个长方形，长方形的长（不是圆柱体的高）是18.84厘米，求这个圆柱体的高（每立方厘米钢重7.8克）．

They considered ______a computer, which was considered ______a great help in their work．

A．to buy; to be

B．buying; being

C．to buy; being

D．buying; to be