Question

If n is a positive integer, which of the following CANNOT be the units digit of $3^n$ ?

• A. $1$
• B. $3$
• C. $5$
• D. $7$

Answer 1

The correct option is C $5$

The units place of $3^1$ is $3$.

The units place of $3^2$ is $9$.

The units place of $3^3$ is $7$.

The units place of $3^4$ is $1$.

The units place of $3^5$ is $3$.

The units place of $3^6$ is $9$.

The units place of $3^7$ is $7$.

The units place of $3^8$ is $1$.

The units place of $3^9$ is $3$.

Now we can clearly observe here that the units place of $3^n$ belongs to the set $3,9,7,1.$

The answer is option $C$ ($5$).