Inertia and restraint systems: when the car suddenly decelerates, your body wants to keep moving forward. A seat belt supplies the restraining force that holds you in place, turning that forward motion into a controlled stop. By keeping you seated, the belt spreads the stopping forces across stronger parts of your body, like the pelvis and torso, which reduces the risk of contacting the interior or being ejected. It also helps position you so airbags can cushion you effectively. That’s why the best choice is that seat belts counteract the effects of inertia by keeping you in your seat. They don’t help you turn faster, they do provide a safety benefit, and they don’t increase injury risk—in fact, they reduce it when worn correctly.

Friction is the force that opposes sliding between two surfaces that are in contact. It arises from the roughness and microscopic adhesion at the interface, and it acts parallel to the contact surface, opposite to the direction of motion or the tendency to move. There are two main kinds: static friction, which prevents motion up to a maximum limit, and kinetic (sliding) friction, which acts once the surfaces are moving relative to each other and is typically smaller in magnitude. The strength of friction depends on how hard the surfaces press together (the normal force) and on the materials and surface textures, summarized by the coefficient of friction. Friction is what lets you walk without slipping, lets brakes slow a car, and makes gears resist motion. Note that gravity is a separate force that can press surfaces together, and air resistance is a different force from a fluid. Energy lost as heat is a consequence of friction doing work, not the definition of friction itself.

The most effective way to prevent transmission is hand hygiene. Hands are the primary vehicle for spreading germs during routine care and daily activities, touching surfaces, and interacting with others. When you clean your hands properly—wash with soap and water or use an alcohol-based hand rub when hands aren’t visibly dirty—you remove or kill many pathogens before they can be passed to patients, coworkers, or surfaces. Because hands encounter so many potential sources of contamination, hand hygiene has the greatest impact on reducing transmission across a wide range of pathogens and settings. Other measures help, but they don’t replace the need for clean hands: gloves don’t substitute for hand cleaning and can create a false sense of security, prophylactic antibiotics don’t prevent transmission and can drive resistance, and isolating every patient is not practical or necessary. Prioritize clean hands to achieve the largest reduction in transmission risk.

Health equity in access to care is about how a range of social and economic factors shape whether people can obtain and benefit from health services. The best statement recognizes that access isn’t determined by a single factor but by multiple interconnected ones—socioeconomic status, race/ethnicity, language, geography, and cultural barriers—that affect whether people actually use care and what outcomes they experience. These determinants influence affordability, availability of services, how comfortable people feel using them, and how care is delivered, leading to disparities in who gets care and how well it works for them. Other options focus on just one factor or on an unrelated factor, which misses the real picture of why access varies across populations.