Under the Inland Navigation Rules, two power-driven vessels have reached a passing agreement via bridge-to-bridge radiotelephone in a crossing situation. Which statement correctly describes their whistle signal obligations?

1. A. Whistle signals are prohibited once a radiotelephone agreement is reached
2. B. Whistle signals are still mandatory and must be exchanged regardless of any radiotelephone agreement
3. C. The vessel that initiated the radiotelephone call must still sound the appropriate whistle signal, but the other vessel is not obliged to respond
4. ✓ The vessels are not obliged to sound the prescribed whistle signals, but may do so; if agreement is not reached, whistle signals shall be exchanged in a timely manner and shall prevailCorrect