Consider the general Reynolds Transport Theorem. The first term on the right hand side represents:
$$\frac{DB_{sys}}{Dt} = \frac{\partial}{\partial t} \int_{CV} \rho b dV + \int_{CS} \rho b \vec{V} \cdot \vec{n} dA$$
Integral over control surface, represents net flowrate of parameter B across entire control surface
Time rate of change of extensive parameter of system
None of these are correct
Rate of change of B within CV as fluid flows through it
Rate of change of B within system as fluid flows through it