A very long (length L) cylindrical galaxy is made of uniformly distributed mass and has radius R (R≪L). A star outside the galaxy is orbiting the galaxy in a plane perpendicular to the galaxy and passing through its centre. If the time period of star is T and its distance from the galaxy's axis is r, then:
a. T∝r
b. T∝r
c. T∝r2
d. T2∝r3