Yeast and sugar are added to water, the gas produced is captured, and its volume is recorded. which variables should be held constant during this investigation?

i know the answer, just need an explanation as to why

The amount of yeast, sugar and volume of water used - to ensure the results are unbiast. ONE of the things above should be changed, to ensure there are differences that occur, but the other two must be maintained.

Were is the question

the glands which are associated with each description is given below:

pancreas - is an endocrine and exocrine gland that secretes juices with enzymes for digestion. salivary glands- with protection and digestion in the mouth. liver- secretes bile for digestion.   lachrymal glands - secrete tears to moisten eyes and protect them from particles and infection.   sweat gland -regulates water, salt, and temperature. oil glands - to form a protective barrier on the skin.   goblet cells - secrete mucus for protection.