(i) A set of odd natural numbers divisible by 2 is a null set because no odd number is divisible by 2.(ii) A={2}.A set of even prime numbers is not a null set because 2 is an even prime number.(iii) {x:x is a natural number,x<5  and x>7} is a null set because a number cannot be simultaneously less than 5 and greater than 7.(iv) {y:y is a point common to any two parallel lines} is a null set because parallel lines do not intersect. Hence, they have no common point.