Four Waves Are Described by the Following Equations,where Distances Are

Multiple Choice

Four waves are described by the following equations,where distances are measured in meters and times in seconds.
I.y = 0.12 cos(3x - 21t)
II.y = 0.15 sin(6x + 42t)
III.y = 0.13 cos(6x + 21t)
IV.y = -0.23 sin(3x - 42t)
Which of these waves have the same period?

A) I and III,and also II and IV
B) I and IV,and also II and III
C) I and II,and also III and IV
D) All of them have the same period.
E) They all have different periods.