Four Waves Are Described by the Following Equations, Where Distances

Multiple Choice

Four waves are described by the following equations, where distances are measured in meters and times in seconds.

Which of these waves have the same period?

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