They would find out by studying hard and researching different things on which is more important
Answer:
That is a compound. If it was an element it would either just be Na or Cl.
This is a combustion reaction because it is being combined with oxygen. Then to balance you will put the number of carbons you have or 4 in front of the CO2 and then same with H2. Then you will add up the oxygen and divide it by two to get. 1, (13/2), 4, 5
The plants without any fertiliser could be the control, to see what the change in the independent variable (fertiliser) does to the growth of the plant (dependant variable).
Hope that helps.
