单项选择题

Passengers are ( ) to through security check. A.
required B.
require C.
requiring D.
requires

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How did the author feel about the sunlight when he was in the hospital?
A、He was amazed at the beauty of the sun.
B、He felt the same as he felt before.
C、He was in a light-hearted mood.
D、He felt it would be better to let it go.

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在反馈控制系统中，设定值如果按照某一函数规律变化，则称为

A．函数控制
B．定值控制
C．程序控制
D．随动控制