Which of the following are sets ? Justify your answer.

The collection of all natural numbers less than $100 .$

Write down all the subsets of the following sets

$\{ 1,2,3\} $

If $A = \{ 1,\,2,\,3,\,4,\,5\} ,$ then the number of proper subsets of $A$ is

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 intervals in set-builder form :

$\left( { - 3,0} \right)$

Consider the sets

$\phi, A=\{1,3\}, B=\{1,5,9\}, C=\{1,3,5,7,9\}$

Insert the symbol $\subset$ or $ \not\subset $ between each of the following pair of sets:

$\phi \,....\,B$