What is hydraulic zero and mechanical zero?

"Zero zero" is not a mathematical operation. One application of the term is in aviation safety, where it describes a type of ejection seat that will take the pilot clear of the aircraft at zero...

Zero is nothing which equals nothing :)

yes

The answer is neither. You cannot divide by zero at all.The result of zero divided by zero, as with any other division by zero, is undefined.

Correct.