Four traveling waves are described by the following equations, where all quantities are measured in SI units and y represents the displacement
I :y=0.12cos(3x−21t)
II : y=0.15sin(6x+42t)
III : y=0.13cos(6x+21t)
IV=y=−0.27sin(3x−42t)
Which of these waves have the same period?
A) I and II, and also III and IV
B) I and IV, and also II and III
C) I and III, and also II and IV
D) All of them have the same period
E) They all have different periods