B. One
because an independent variable is something you change. So in a scientific experiment you only change one thing. If you happen to change more than one thing then the test becomes unfair therefore you should repeat and make sure that only one thing is changed this time.
I believe the answer to 11 is that multicellular organisms are made up of multiple cells and unicellular organisms are made up of one cell.
I think it’s B
Hope that’s right
B.the mass of carbon dioxide plus water is less than that of paper plus oxygen.