A racing car moves along a circular track of radius $$b$$. The car starts from rest and its speed increases at a constant rate $$\alpha$$. Let the angle between the velocity and the acceleration be $$\theta$$ at time $$t$$. Then $$\cos\theta$$ is

A

0

B

$$\frac{\alpha t^2}{b}$$

C

$$\frac{b}{b + \alpha t^2}$$

D

$$\frac{b}{(b^2 + \alpha^2 t^4)^{1/2}}$$