多项选择题

下列哪些是相对数( )。 A.
频数 B.
频率 C.
均值 D.
环比

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单项选择题

Federal reserve assets include A.
government securities. B.
bank reserves. C.
currency in circulation. D.
all of the above.

单项选择题

Why do UAV cameras special? These cameras are _____.

A．expensive
B．unmanned aerial vehicles.
C．vehicles specially made for military use
D．modern optical experimental systems