Before changing lanes or attempting to pass another vehicle, you should:

Answer: C — All the above

Before changing lanes or attempting to pass another vehicle, it is essential to both check your blind spots, mirrors, and look over your shoulder toward the rear, and also signal your intention before changing lanes. By doing both these actions, you ensure that you are aware of your surroundings and you communicate your intentions to other drivers on the road. This helps to prevent accidents and promotes safer driving practices overall. Therefore, the correct answer is C, as it includes both of these important steps for changing lanes safely and effectively.