The correct answer is A) enter the riverward lock. Two long blasts of the horn from the lock indicate that the lock is ready to receive upbound vessels. As the vessel holding position above the lock, you should proceed to enter the riverward lock in this situation. The other options are incorrect because: B) holding position until the lower gates are closed is not the appropriate response to the two long blasts signal, C) entering the landward lock would be the incorrect direction for an upbound vessel, and D) holding position until two more upbound tows have locked through is not the correct response to the two long blasts signal.