Air resistance dominates frictional effects at higher speeds

Steps:

• Identify main frictional forces on a cyclist: rolling friction (tyre-road) and air resistance (drag).
• Note rolling friction depends on weight and surface, but stays roughly constant with speed.
• Recognize air resistance increases significantly with velocity, as per drag equation.
• Conclude greater speed amplifies total frictional opposition to motion.

Why B is correct:

• Air drag force $F_d = \frac{1}{2} \rho v^2 C_d A$ increases with the square of speed $v$, per fluid dynamics.

Why the others are wrong:

• A: Lighter bicycle reduces normal force, decreasing rolling friction.
• C: Smoother tyres lower coefficient of friction, reducing rolling resistance.
• D: Oiled road decreases surface friction coefficient, easing motion.

Final answer: B