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 sets ? Justify your answer.
The collection of all natural numbers less than $100 .$
Which of the following is a true statement
Write down all the subsets of the following sets
$\{ a\} $
Make correct statements by filling in the symbols $\subset$ or $ \not\subset $ in the blank spaces:
$\{a, b, c\} \ldots\{b, c, d\}$