Four Waves Are Described by the Following Expressions, Where Distances

Multiple Choice

Four waves are described by the following expressions, 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 travel in the +x direction?

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