土方回填时，( )。

A．不同种类的土可以混杂在一起填筑

B．不同种类的土不能混在一起填筑

C．不同种类的土可以按适当比例混杂填筑

D．以上作法都可以

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