The scalar field for the evolution of concentration diffusing from a point source in three dimensions is c(r,t)=(4Dt)3/2ndexp(4Dtr2), where nd is the number of particles in the point source and D is the diffusion coefficient. a. Find the expression for the vector field J(r,t)=Dc, where J(r,t) is the diffusion flux. Use spherical coordinates. b. Assume that matter is conserved during diffusion and find an expression for the rate of the accumulation of diffusing particles at any point. c. In words, describe the general trend for how the rate of accumulation varies with time at a fixed point r=0. Consider times from close to t=0 out to a time that is sufficiently long for the concentration field to become essentially negligible at the fixed point r