Let $A=\{1,2,3,4,5,6\} .$ Insert the appropriate symbol $\in$ or $\notin$ in the blank spaces:

$ 5\, .......\, A$

Which of the following are sets ? Justify your answer.

The collection of all the months of a year beginning with the letter $\mathrm{J}.$

Which of the following are examples of the null set

Set of odd natural numbers divisible by $2$

Which of the following are sets ? Justify your answer.

A collection of most dangerous animals of the world.

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