The correct answer is C) may cause frothing. Antifreeze solutions containing ethylene glycol should not be mixed with corrosion protection oils because the resultant mixture may cause frothing. The chemical reaction between the antifreeze and the oils can create a foamy, frothy mixture that can interfere with the proper functioning of the system. This frothing can disrupt normal fluid flow and potentially damage components. The other answer choices are incorrect, as the mixture does not become dangerously flammable, have a higher chloride content, or promote scale buildup.