18. One of the fundamental laws of classical electromagnetism is Gauss' law for magnetic fields, which says that a magnetic field is a vector field B : R3 - R' satisfying V . B = 0 everywhere. Define a vector field F on R' to be "magnetic" or a "magnetic field" just if V . F = 0. a) Use the divergence theorem to evaluate the surface integral: F . Nas where F(x, y, z) = yzj and S is the cylinder x2 + y? = 9, 0