Which of the following sets are empty sets and which are singleton sets ?
(i) Set of even prime numbers
(ii) `{x : x +2 = -1 and x in N}`
(iii) Set months starting with D
(iv) `(x : x " is a common point in two parallel lines")`.

Explanation

(i) 2 is the only number in the set of even prime numbers
`:.` Required set `= {2}`
which is a singleton set
(ii) `x +2 = -1`
`rArr x = -3` which is not a natural number.
`:.` In the set of natural numbers, given equation has no solution
`:. {x:x + 2 = -1 and x in N}={ }`
which is empty set
(iii) Set of months starting with D = {December} which is singleton set
(iv) There is no common point in two parallel lines.
`:.` Given set is empty set.