单项选择题

Which of the following descriptions is NOT correct about Benjamin Franklin?

A．He invented bifocal glasses for the elderly.
B．He designed the earliest swimming spectacles and flippers.
C．He created 9 and 19 magic squares.

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（）是坚持党对人民军队绝对领导的根本制度和根本实现形式，在党领导军队的一整套制度体系中处于最高层次、居于统领地位。

A.军委主席负责制
B.首长分工负责制
C.政治委员制
D.支部建在连上

单项选择题

What is the cross-correlation of a 10 Hz square wave and a 20 Hz sinusoidal wave?

A．Zero line
B．10 Hz sinusoidal wave
C．20 Hz sinusoidal wave
D．A signal with complicated waveform