And that is what this is a scalar function, so it is not always it is not necessary to that only we can talk about d by d t the total derivative. In case of a vector function we can also talk about the material derivative in case of a solar function provided, they have the similar characteristics and property of the fluid. Now, this background again yesterday I had talked about that how to convert using the gauss diversion theorem, I have told that we can always sense a surface integral to line integral. And this equation will help us, this theorem gauss diversion theorem will help us in deriving the equation of continuity.