Write the following intervals in set-builder form :

$\left[ {6,12} \right]$

Which of the following are sets ? Justify your answer.

A team of eleven best-cricket batsmen of the world.

Which of the following are examples of the null set

Set of odd natural numbers divisible by $2$

Let $A=\{1,2,\{3,4\}, 5\} .$ Which of the following statements are incorrect and why ?

$\{\varnothing\} \subset A$

In each of the following, determine whether the statement is true or false. If it is true, prove it. If it is false, give an example.

If $A \subset B$ and $x \notin B,$ then $x \notin A$