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【单选题】
自相关函数是区别信号类型的一个有效手段，若信号中含有周期成分，则其自相关函数______。A.
在τ很大时衰减也很小
B.
在τ很小时衰减就很大
C.
在τ很大不衰减并具有明显的周期性
D.
在τ很大时不衰减但不具有明显的周期性
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皮皮学刷刷变学霸
举一反三
【单选题】He told us that he would leave for Shanghai ( ).
A.
next day
B.
the next day
C.
the future day
【单选题】He told us ________ story that all of us were pleased to hear it.
A.
so interesting
B.
such interesting
C.
such an interesting
D.
so an interesting
【简答题】精密称定系指称取重量应准确至所取重量的（）；称定系指称取重量应准确至所取重量的（）。
【单选题】He told us that he ________ for two hours.
A.
hasbeen waiting for us
B.
had been waiting for us.
C.
is waiting for us
D.
waiting for us
【单选题】He told us that he was tired and thirsty because he ________ in the desert for days.
A.
walked
B.
has walked
C.
had walked
D.
would walk
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