Here's a quiz for you DIY'ers

[X5only]

Ok, you expert DIY'ers, here's a lil' quiz for ya. Look at the picture; now what do you think caused this? The expansion tank is brand new.

Choose the correct answer below. Only one answer is correct:

A) The engine overheated expanding the coolant beyond what the cooling system could handle;

B) The expansion tank has a manufacturers defect;

C) The coolant was overfilled causing over-pressurization of the cooling system;

D) The coolant got extremely hot thus melting the expansion tank joints.