The necessary condition for a number to be rational is that it can be expressed as an integer divided by a non-zero integer. This means that a rational number can be written in the form \( \frac{a}{b} \), where \( a \) is an integer and \( b \) is a non-zero integer.
While it's true that rational numbers can also be expressed as fractions, the key defining feature is the ability to be written in the form of a ratio of two integers. Endless decimals are generally associated with irrational numbers unless they are repeating decimals, which are actually rational. Additionally, rational numbers can be both positive and negative, so the requirement of not being negative is incorrect.
So the correct answer is: it must be able to be expressed as an integer divided by a non-zero integer.