Write down all the subsets of the following set: $\{a\}$

Let $A = \{1, 2, 3, \ldots, 10\}$ and $B = \left\{\frac{m}{n} : m, n \in A, m < n \text{ and } \gcd(m, n) = 1\right\}$. Then $n(B)$ is equal to:

Which of the following are examples of the null set?
Set of even prime numbers

In this question,all integers are represented in base $10$. Consider the set $E$ of positive integers $n$ having the property that when any nonzero digit $d \in \{1, 2, \dots, 9\}$ is written to the right of $n$,the resulting number is divisible by $d$. Let $N$ be the smallest element of $E$. The product of the digits of $N$ is:

If $R$ is the set of real numbers and $Q$ is the set of rational numbers,then what is $R - Q$?

Which of the following are sets? Justify your answer.
$A$ collection of the most dangerous animals of the world.