A man is watching two trains, one leaving and the other coming with equal speed of 4 m/s. If they sound their whistles each of frequency 240 Hz, the number of beats per sec heard by man will be equal to: (velocity of sound in air=320 m/s):
| A. |
12 |
| B. |
zero |
| C. |
3 |
| D. |
6 |
 |