It is common knowledge that a given word can have various

It is common knowledge that a given word can have various shades of meaning, but how many people are aware that a single word can sometimes be used in completely contradictory ways? For example, the word "cleave" can mean "to adhere" or "to stick together." But just as easily the word "cleave" can mean "to split apart" or "to divide." This proves that words are fundamentally meaningless until they are placed in a sentence.

Which one of the following is an assumption necessary to the author's argument?

Every sentence is meaningful.
Common knowledge is usually wrong.
The possession of two contradictory meanings is equivalent to an absence of meaning.
The word "cleave" is unique in that it can be used in completely contradictory ways.
A word that possesses two shades of meaning is a word that possesses two contradictory meanings.