42. The current density in a particle beam with circular cross section of radius a points along the beam axis with a magnitude that decreases linearly from ?0 at the center (r = 0) to half that value at the edge (r = a). What is the total current in the beam? (A) 13 ?0?a2 (B) 12 ?0?a2 (C) 23 ?0?a2 (D) 56 ?0?a2 (E) 16 ?0?a2