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

1. Ball bearings are components used to reduce rotational friction and support loads between moving parts, such as between a wheel and an axle in bicycles, cars, etc. 2. They consist of smooth metal balls rolling between two rings (races). 3. The primary function of ball bearings is to replace sliding friction (which occurs when surfaces slide directly against each other) with rolling friction. 4. Rolling friction is significantly lower than sliding friction. This reduction in friction is achieved because the contact between the rolling balls and the races occurs over a very small area (theoretically a point or a very small ellipse under load). 5. Compared to a plain bearing where the entire surface of the axle might slide against the wheel hub (or vice versa), the effective area of contact through which the load is transferred and friction occurs is drastically reduced in a ball bearing system. 6. Options (A) and (B) are incorrect; the use of rolling elements (balls) reduces, rather than increases, the effective area of contact compared to sliding surfaces. 7. Therefore, the reason ball bearings are used is that they reduce the effective area of contact, thereby replacing high sliding friction with low rolling friction.