Final answer:
If lowered crossbars are present at a railroad crossing, the correct action is to stop and wait until the train has passed and the crossbars are raised.
Step-by-step explanation:
When you see lowered crossbars at a railroad crossing, you should stop moving and become stationary till the train passes by. It is the only safe and appropriate action to take in this situation. Lowered crossbars are a clear signal that a train is approaching, and it is crucial for your safety to remain behind the crossbars until they are raised and it's clear to proceed.