List all the elements of the following set:
$A = \{ x:x \text{ is an odd natural number} \}$

Which of the following is a set? Justify your answer.
The collection of all boys in your class.

Which of the following is the empty set?

If $Q = \{x : x = \frac{1}{y}, y \in N\}$,then which of the following is true?

Two sets $A$ and $B$ are disjoint if and only if

In a city,no two persons have an identical set of teeth,and there is no person without a tooth. Also,no person has more than $32$ teeth. If we disregard the shape and size of the teeth and consider only the presence or absence of a tooth at each of the $32$ positions,then the maximum population of the city is: