A two-part sand mould must contain a riser. What is the primary function of the riser?
A. to enable the flow of the melted metal through the actual mould at a low speed to minimize the porosity in the mould
B. to enable the easy removal of the core without destruction of the cope when the mould is raised
C. to feed the mould cavity during solidification
D. to perform the function of the runners within the vertical section of the mould