When taking a tissue culture for thoes who have cancer it allowes for more of a trial error method. When a tissue culture is collected the scientist/doctors get to test procedures on it to see what teatment works best for you. With out them there would be many people taking medicins that dont even help them with their condition; and in few cases could make it worse. Just saying tissue culturs' are a huge advancement in technology, and science.
It is the manipulated variable. It is changed in an experiment to see how it would effect another variable.
I think it is C as plants take in Co2 and release oxygen which humans use to breathe.
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