PLEASE HELP ME I DONT UNDERSTAND THIS AND THIS IS TIMED
In an experiment, a rubber stopper is attached to one end of a string. The stopper is whirled in a horizontal circular path of a diameter of 1.0 meter at a constant speed. The stopper completes one revolution in 0.2 seconds. Approximately what is the magnitude of the stoppers speed?

A) 3.1 m/s
B) 6.3 m/s
C) 16 m/s
D) 31 m/s