在拓扑结构上，快速以太网采用（ ）。

A．总线型拓扑结构

B．环型拓扑结构

C．星型拓扑结构

D．树型拓扑结构

But for their help，we ______ the program in time.

A．can not finish

B．will not finish

C．had not finished

D．could not have finished