Answer:
a or d
Irrational numbers are numbers that cannot be expressed as the ratio of two whole numbers. This is opposed to rational numbers, like 2, 7, one-fifth and -13/9, which can be, and are, expressed as the ratio of two whole numbers. When expressed as a decimal, irrational numbers go on forever after the decimal point and never repeat.
The Greek mathematician Hippasus of Metapontum is credited with discovering irrational numbers in the 5th century B.C., according to an article from the University of Cambridge. While working on a separate problem, Hippasus is said to have stumbled on the fact that an isosceles right triangle whose two base sides are 1 unit in length will have a hypotenuse that is √2, which is an irrational number. (This can be shown using the famous Pythagorean theorem of a^2 + b^2 = c^2.)
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