Final answer:
The pairs of quantities that are proportional to each other in a circle are the radius and diameter, radius and circumference, and radius and area.
Step-by-step explanation:
The quantities that are proportional to each other in a circle are:
- a. Radius and diameter: The diameter is always twice the length of the radius. So, if you double the radius, the diameter will also double.
- b. Radius and circumference: The circumference of a circle is equal to 2π times the radius. So, if you double the radius, the circumference will also double.
- c. Radius and area: The area of a circle is equal to π times the square of the radius. So, if you double the radius, the area will increase by a factor of four.
Therefore, the correct pairs of quantities that are proportional to each other are a, b, and c.