把某一项投资活动最为一个独立的系统，其资金的流向、数额和()都不仅相同。

A.周期

B.时间长度

C.发生时点

D.现金流量

Armstrong became interested in _______ ever since he took his first _______ at the age of six.

A．fly; flying

B．flight; fly

C．flight; flying

D．flying; flight