An Ideal Solenoid of Radius a Has N Turns Per $\phi$

An ideal solenoid of radius a has n turns per unit length and current I.The magnetic flux $\phi$ _B through any circular area of radius a inside the solenoid,centered on and perpendicular to the solenoid axis is

A) $\mu _ { 0 } \frac { \pi a ^ { 2 } } { 4 } n I$ .
B) $\mu _ { 0 } \frac { \pi a ^ { 2 } } { 2 } n I$ .
C) $\mu$ ₀ $\pi$ a²nI.
D) 2 $\mu$ ₀ $\pi$ a²nI.
E) 0.