In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
Spread spectrum simply means that data is sent in small pieces over a number of the (16) frequencies available for use at any time in the specified range. Devices using (17) spread spectrum(DSSS) communicate by (18) each byte of data into several parts and sending them concurrently on different (19) . DSSS uses a lot of the available (20) , about 22 megahertz(MHz).

A．rate

B．velocity

C．bandwidth

D．period

决策树法的要素有（）。

A.决策点

B.方案枝

C.状态节点

D.概率枝