问题
选择题
Several minutes after its launch, the X-37B plane successfully sent back its first___________to the control center.
A．mark
B．sign
C．instruction
D．signal
答案
答案：D
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Several minutes after its launch, the X-37B plane successfully sent back its first___________to the control center.
A．mark
B．sign
C．instruction
D．signal
答案：D