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

question 7

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.