Write the following sets in roster form :
$C = \{ x:x{\rm{ }}$ is a two-digit natural number such that sum of its digits is $8\} $
Write the following sets in roster form :
$\mathrm{F} =$ The set of all letters in the word $\mathrm{BETTER}$
Write the set $\{ x:x$ is a positive integer and ${x^2} < 40\} $ in the roster form.
Which of the following are examples of the null set
Set of odd natural numbers divisible by $2$
List all the elements of the following sers :
$B = \{ x:x$ is an integer $; - \frac{1}{2} < n < \frac{9}{2}\} $