Final answer:
Flight attendants must repeatedly shout the command 'Exit Quickly!' to passengers when opening the aircraft's exit door during an evacuation.
Step-by-step explanation:
When opening the aircraft's exit door to evacuate, flight attendants must repeatedly shout to the passengers the command Exit Quickly! The command is given to ensure that passengers leave the aircraft as quickly as possible during an emergency situation. It is important for passengers to exit quickly to minimize the risk of injury or further harm. By shouting this command, flight attendants help to communicate the urgency of the situation and direct passengers to evacuate promptly and safely.