●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 (66) pieces over a number of the discrete frequencies available for use at any time in the specified range．Devices using (67) spread spectrum(DSSS)communicate by splitting each (68) of data into several parts and sending them concurrently on different (69) ,DSSS uses a lot of the available (70) ，about 22 megahertz(MHz)．

(66) A．small

B．high

C．low

D．long

(67) A．direct-sequence

B．discrete-sequence

C．duplicate-sequence

D．dedicate-sequence

(68) A．data

B．byte

C．packet

D．bit

(69) A．bits

B．frequencies

C．packets

D．messages

(70) A．rate

B．velocity

C．bandwidth

D．period