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A circle with a diameter of 18 would have a circumference of 18π, not 18. This is because the circumference of a circle is equal to the diameter multiplied by π (pi), not just the diameter itself. Additionally, the area of a circle with a diameter of 18 would be 81π, not just 9π. Therefore, options A and C are incorrect. Option B states that the circumference is 18, which is incorrect because the diameter is 18 and the circumference would be larger than the diameter. Option D correctly states that the circumference of a circle with a diameter of 18 is 9π.