Cosmology

The diagram shows a source of sound waves and an observer.

Which row of the table shows a situation which would result in a decrease in the frequency of sound observed?

	Source	Observer
A	moves to the right at 20 ms−1	moves to the left at 20 ms−1
B	moves to the right at 20 ms−1	moves to the right at 20 ms−1
C	moves to the right at 20 ms−1	stationary
D	stationary	moves to the right at 20 ms−1