# SAT Math Multiple Choice Practice Question 474: Answer and Explanation

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**Question: 474**

**15.** If the circle with center *O* has a diameter of 9, then what is the area of the circle with center *O* ?

A. 81π

B. π

C. π

D. 18π

E. 9π

**Correct Answer:** C

**Explanation:**

**C** The formula for the area of a circle is , so you need to find the radius, *r,* of the circle. You know that the diameter is 9, and the radius is half of the diameter, so the radius is . Because there are fractions in the answer choices, you might as well keep the radius as a fraction. Now you replace *r* with in the formula for area of a circle: .