The magnetic field in a region is radial and varies with the distance r from a fixed point O according to B(r)=B0Rr for 0≤r≤R, where B0 and R are constants. The field is everywhere perpendicular to a flat circular surface of radius R centred at O. The total magnetic flux through the surface is
(a) 2πB0R2
(b) 32πB0R2
(c) πB0R2
(d) 3πB0R2