What is the main idea of this passage?

Câu hỏi :

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Ever since human inhabited the earth, they have made use of various forms of communication. Generally, this expression of thoughts and feelings has been in the form of oral speech. When there is a language barrier, communication is accomplished through sign language in which motions stand for letters, words, or ideas. Tourists in foreign countries, for example, the deaf, and the mute have had to resort to this form of expression. Many of these symbols on the whole are very interesting and exact, many can be used internationally; however, some can be confused for different requests or expressions.

Body language transmits ideas or thoughts by certain actions, either intentionally or unintentionally. A wink can indicate that person is only joking. A nod signifies approval while shaking the head indicates a negative reaction.

Other forms of non - linguistic language can be found in Braille (a system of raised dots on paper read with finger tips), signal flags, Morse Code and smoke signals. Road maps and picture signs also guide, warn or instruct people. While verbalization is the most common form of language, there are other systems and techniques which express human thoughts and feelings.

What is the main idea of this passage?