Ball bearings are used in bicycles, cars, etc., because

The correct answer is C. Ball bearings reduce friction by cutting down the effective area of contact between the moving parts. A ball touches the race at a very small point, so the surfaces roll over each other instead of sliding. Rolling friction is far smaller than sliding friction, which is why the wheel turns easily.

• A is wrong. Ball bearings do not enlarge the actual contact area. Their whole design aims at minimal contact.
• B is wrong. A larger effective contact area would mean more friction and more wear, the opposite of the purpose.
• D is wrong because C is correct.

The core concept is the difference between sliding and rolling friction, a basic idea in engineering mechanics and tribology. For Mains, it links to machine design, energy efficiency, and lubrication. Lubricants and bearing materials work alongside this principle.

Trap: Aspirants often confuse the geometric (actual) contact area with the effective area where the surfaces really touch. Read the stem for what the design is trying to achieve.