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单选题
Which of the following phrases is allowed?
A
“when passing” + “slot/time/significant point/level” + “contact” + “unit” + “frequency”
B
“when passing/maintain” + “level” + “direct to” + “waypoint”
C
“when traffic clear” + “climb/descent to” + “level”
D
“when ready” + “climb/descent to” + “level”
答案解析
正确答案：B
