Which of the following are examples of the null set

$\{ x:x$ is a natural numbers, $x\, < \,5$ and $x\, > \,7\} $

Which of the following are examples of the null set

$\{ y:y$ is a point common to any two parallellines $\} $

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:

$B \ldots \cdot C$

Write the set $\{ x:x$ is a positive integer and ${x^2} < 40\} $ in the roster form.

List all the elements of the following sers :

$E = \{ x:x$ is a month of a year not having $ 31 $ days ${\rm{ }}\} $

Assume that $P(A)=P(B) .$ Show that $A=B$.