flat ball pair的用法和样例：
- Whether it was a flat ball down the line or a heavy topspin angle shot, I was able to go after each shot with confidence because of the control I was getting.
- In computer graphics, a hand-held locator operated by moving it on a flat surface.A mouse generally contains a control ball or a pair of wheels.
- The man jacked up his car to fix a flat tire.
- The Low Temperature-sensitive Coefficient Charge of Flat Ball Gun Propellant
- He stumbled over a stump and fell flat on his face.