Which of the following are examples of the null set?
(i) Set of odd natural numbers divisible by $2$
(ii) Set of even prime numbers
(iii) \left \{ x : x \text { is a natural numbers, x < 5$and$x > 7}\right \}$(iv)$\left \{ y : y \text { is a point common to any two parallel lines}\right \}$$

(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) $\left\{x:x\text{ is a natural number},x<5\text{and}x>7\right\}$ is a null set because a number cannot be simultaneously less than $5$ and greater than $7$.

(iv) $\left\{y:y\text{ is a point common to any two parallel lines}\right\}$ is a null set because parallel lines do not intersect. Hence, they have no common point.