单选题
To identify planes flying overhead, the U. S. air-traffic controllers used to _____.
A、
(A) get information by communication through phone calls
B、
(B) depended on the "war room" for information
C、
(C) collect data by way of radar
D、
(D) use several computers working at the same time
【正确答案】
D
【答案解析】
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