Mass per unit area of a circular disc of radius a depends on the distance r from its centre as σ (r) = A + Br The moment of inertia of the disc about the axis, perpendicular to the plane and passing through its centre is :
A. 2πa⁴ (A / 4 + aB / 5)
B. 2πa⁴ (aA / 4 + B / 5)
C. πa⁴ (A / 4 + aB / 5)
D. πa⁴ (A / 4 + B / 5)