47. According to the passage, what kind of signal has long periods of light that are regularly broken by two or more periods of darkness?
(A) Group occulting.
(B) Flashing.
(C) Occulting.
(D) Group flashing.

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統計: A(95), B(15), C(13), D(27), E(0) #593190

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#3922298
燈塔是具有強光的塔,可幫助水手們確定位置...
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#6026493
對照文章中的這一句 "a group-o...
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