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