Which One of the Following Superpositions Will Result in Beats

Multiple Choice

Which one of the following superpositions will result in beats?

A) the superposition of waves that travel with different speeds
B) the superposition of identical waves that travel in the same direction
C) the superposition of identical waves that travel in opposite directions
D) the superposition of waves that are identical except for slightly different amplitudes
E) the superposition of waves that are identical except for slightly different frequencies