Assertion: According to big-bang hypothesis about 20 billion years ago universe was a big ball of only neutrons.
Reason: Movement of these particles is known to generate tremendous heat which caused explosion due to temperature and pressure changes resulting in formation of universe.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

Big bang theory explains the origin of universe, according to this hypothesis about 20 billion years ago the universe was the big ball made up of neutrons. Its huge explosion due to its high temperature change in pressure, the universe was formed.