Final answer:
If you hear sirens but cannot see where the emergency vehicle is coming from, the best course of action is to stop moving and become stationary until it passes by.
Step-by-step explanation:
If you are certain that you hear sirens but cannot see where the emergency vehicle is coming from, the best course of action is to stop moving and become stationary until it passes by. This is because it is important to give priority to emergency vehicles and allow them to pass through without any hindrance. Running toward or alongside the ambulance can be dangerous and can obstruct the emergency response. So, it is always recommended to stay stationary and let the emergency vehicle pass.