19. let f(x, y) = and let c be the upper half of the unit circle. which of the following is not equivalent to sf. dr? xydx + x²ydy b) sint cost dt dr 20. which theorem is useful in physical problems which require the computation of the net total flux of a vector field across a closed surface? a) the fundamental theorem for line integrals b) green's theorem c) the divergence theorem d) stokes' theorem