(i) Set of odd natural numbers divisible by 2 is a null set because odd natural numbers are not divisible by 2. (ii) Set of even prime numbers is {2} which is not a null set. (iii) {x : x is a natural number, x < 5 and x >7} is a null set because there is no natural number which satisfies x < 5 and x > 7 simultaneously. (iv) {y : y is a point common to any two parallel lines} is a null set because two parallel lines do not have any common point.