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 speed?

A) I and II
B) I and III
C) II and III
D) III and IV
E) II and IV