机械天平(电光分析天平)若长时间不使用,则应定时通电预热,一般每()一次,每次预热2h,以确保仪器始终处于良好使用状态。
A. 3天
B. 7天
C. 15天
D. 30天
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______ is used for providing homing signals from survival craft for detection by 9 GHz radar.
A. NAVETEX receiver
B. search and rescue transponder
C. emergency position indicating radio beacon
D. digital selective calling