Which of the Following Represents a Standing Wave

Multiple Choice

Which of the following represents a standing wave?

A) y = (6.0 mm) sin[(3.0 m^-1) x + (2.0 s^-1) t] - (6.0 mm) cos[(3.0 m^-1) x + 2.0]
B) y = (6.0 mm) cos[(3.0 m^-1) x - (2.0 s^-1) t] + (6.0 mm) cos[(2.0 s^-1) t + (3.0 m^-1) x]
C) y = (6.0 mm) cos[(3.0 m^-1) x - (2.0 s^-1) t] - (6.0 mm) sin[(2.0 s^-1) t - 3.0]
D) y = (6.0 mm) sin[(3.0 m^-1) x - (2.0 s^-1) t] - (6.0 mm) cos[(2.0 s^-1) t + (3.0 m^-1) x]
E) y = (6.0 mm) sin[(3.0 m^-1) x] + (6.0 mm) cos[(2.0 s^-1) t]