Fear of loooonnnnnggg words
lol
Answer: Extraordinary assumption
Explanation: Appraisal could be defined as the estimation or evaluation of an object, property or structure in other to determine either the worth, condition or nature of such property or object.
An Extraordinary assumption during appraisal means a presumption that certain unknown information exists or is a fact. It assumes a condition which is unknown to be true and as such if the information is found to be false may alter the result or conclusion of the appraisal.
Such is the scenario above, when the appariser assumes there is no leakage or contamination based merely on unknown fact.
Yes, this is true!
You should genereally use sources for anything you say if it's not your original idea - this way you can avoid being accused of plagiarism.
One exception is common knowledge - so you don't have to provide sources for example for what a capital of a certain country is.