The correct answer is C. The main difference between a stuffing box gland and a mechanical seal is that if the packing in a stuffing box gland fails, the leak can be temporarily stopped by tightening the gland. However, if a mechanical seal fails, the entire seal must be replaced to stop the leakage, as there is no way to "tighten" a mechanical seal. The other options are incorrect because: A) Both stuffing box glands and mechanical seals are subject to wear over time. B) Mechanical seals can be designed to operate without external cooling, depending on the application. D) The orientation of the sealing surface is not the primary difference between the two sealing methods.