A stone of mass 2.6 kg is tied to the end of a string and is swung with constant speed around a horizontal circle with a radius of 0.52 m. If it makes two complete revolutions each second, its acceleration is:.
a. 82.11 m/s^2
b. 42.70 m/s^2
c. 5.13 m/s^2
d. 6.54 m/s^2
e. 20.53 m/s^2