A. zero

B. irrational number

Correct answer: b

Three crises in mathematics:

First, Hippasu (5th century BC) discovered that the hypotenuse of an isosceles right triangle with a waist of 1 (i.e. root number 2) can never be expressed by the simplest integer ratio (incommensurable ratio), thus discovering the first irrational number and overthrowing Pythagoras' famous theory. It is said that Pythagoras was at sea at that time, but because of this discovery, Herbs was thrown into the sea.

Second, the rationality of calculus is seriously questioned, which almost subverts the whole calculus theory.

Third, Russell's paradox: S is composed of all elements that do not belong to itself. Does s contain S? In layman's terms, one day Xiaoming said, "I'm lying!" "Ask xiao Ming is lying or telling the truth. The terrible thing about Russell's paradox is that it doesn't involve the profound knowledge of sets like the maximum ordinal paradox or the maximum cardinal paradox. It is simple, but it can easily destroy set theory.