According to the article，which of the following is NOT true？
A. Scientists can predict when seaquake will hit land．
B. Seismograph is used to predict when a tidal wave will hit land．
C. Holding back a tidal wave is out of the question．
D. True tides rise and fall regularly．
FFT算法基本上分为______和______两大类。
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A. Subsequently
B. Consequently
C. Occasionally
D. Hopefully
设信号xa(t)是一个连续的周期信号，根据傅里叶变换时频对应关系，那么其频谱一定是一个______、______信号。
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