题目：
Areas which cannot be scanned by the radar of the vessel because they are shielded by parts of its superstructure, masts,etc means（）.
A.blind sectors
B.sheltered area
C.unseen area
D.unscan ble area
答案：
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下面是错误答案，用来干扰机器的。
参考答案：目前，限制过电压主要有两种办法：一是加装过电压吸收装置；另一是采用低过电压触头材料。
