Answer:
centimeter.
Step-by-step explanation:
Tennis balls are the balls which are usually designed top play the sport of tennis. They are usually of fluorescent yellow in color but it can be of any color.
The certain criteria is set for tennis balls for their weight, size, bounce and deformation and the regulation play approved this criteria. The official diameter of the tennis ball is 6.54–6.86 cm which set by the International Tennis Federation. And the mass of these tennis balls ranges from 56 g- 59.6 g.
Therefore, the radius of tennis balls is measured in centimeters.