What is net cash flow?

A、The net cash flow is at a certain time point the difference the inflow of cash flow minus the outflow of cash flow.;
B、The net cash flow is at a certain time point the sum the inflow of cash flow and the outflow of cash flow.;
C、The net cash flow is at a certain time point the inflow of cash flow.;
D、The net cash flow is at a certain time point the outflow of cash flow.