You are given the diameter as 18 inches. What is half of 18? That value is the radius.

Once you know the radius, substitute it into $A = \pi r^2$. Be careful to square the radius, not to multiply by 2.

In Desmos, type 18/2 and evaluate it to confirm the radius of the circle. You should get a whole number value for the radius.

Next, type (18/2)^2 in Desmos. Note the whole number result; this is $r^2$.

The area is $\pi$ times the value you just found for $r^2$. Think of the area as that number · pi and select the answer choice whose coefficient (the number in front of $\pi$) matches that number.

The diameter of a circle is twice the radius.

Given diameter $d=18$ inches:

• Radius $r$ is half the diameter, so

So the radius of the circle is 9 inches.

The area $A$ of a circle in terms of its radius $r$ is

We will substitute $r=9$ into this formula.

Plug $r=9$ into the formula $A=\pi r^2$:

Now square 9:

So the area is

Therefore, the correct answer is $81\pi$, which corresponds to choice B.