A Beam of Electrons Moves at Velocity

A beam of electrons moves at velocity . The number of particles per unit volume in the beam of area A is ρ. If we imagine a cylindrical Gaussian surface of radius r and length centered on the beam, the electron flux through the surface is

A) 0.
B) ρvf A.
C) 2ρvf A.
D) ρvf (A+2πr ) .
E) 2ρvf (A+πr ) .