Which of the following are sets ? Justify your answer.

A collection of most dangerous animals of the world.

State whether each of the following set is finite or infinite :

The set of lines which are parallel to the $x\,-$ axis

The number of elements in the set $\{ (a,\,b):2{a^2} + 3{b^2} = 35,\;a,\,b \in Z\} $, where $Z$ is the set of all integers, is

Which of the following are sets ? Justify your answer.

The collection of all boys in your class.

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

$\{1,2,5\}\subset A$