People sometimes wonder whether you know ___ body language w

People sometimes wonder whether you know ___ body language works.
下面为原文,供大家参考.
Throughout history people have always communicated with each other not only by speech but also by movement of the hands and body. It is, however, only in the last few years that such communication has been studied widely. This kind of communication is considered to be body language or non-verbal communication.
People sometimes wonder whether you know (答案how,其他选项why, what,where.两个问题1为什么?2此句翻译) body language works. It is of course possible to read books on the subject but you also need to spend time watching people’s movements. A railway station is a particularly good place for such a thing, as there people can be seen openly expressing anger, sorrow, happiness, impatience and many other human emotions by means of movements.
If you turn down the sound on your TV and try to understand what is happening simply by watching the picture, you will learn even more about communication without words. By turning the sound back up very five minutes or so, it is possible to check how exact your understanding is.
Having studied the art of body language, you will have an advantage at a boring party. You will be able to sit on your own for the whole evening and enjoy yourself by both watching and interpreting the body language of all the other people there.
为什么选how ，说明一下，为完形填空题。

人们有时候会想知道肢体语言[是怎样]起作用的
几个选项从语法上都对.从意思上选how.后面讲,怎样起作用,不是为什么起作用,也不是在哪里起作用.