why is Q not on the other side, if we move down (South) from where dh/dx is 0 than also h which decrease  right? I mean dh/dy < 0  for both points right? why only for the upper one?####[['Q', 'Mathematical_Related_Concept', 'NEU'], ['h', 'Mathematical_Related_Concept', 'NEU'], ['dh/dy < 0', 'Mathematical_Related_Concept', 'NEU'], ['dh/dx', 'Mathematical_Related_Concept', 'NEU']]