<u>Answer:</u> The product side must be 
<u>Explanation:</u>
Single displacement reaction is defined as the reaction in which more reactive metal displaces a less reactive metal from its chemical reaction.

Metal C is more reactive than metal A.
The reactivity of metal is determined by a series known as reactivity series. The metals lying above in the series are more reactive than the metals which lie below in the series.
Law of conservation of mass states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form. This also means that total number of individual atoms on reactant side must be equal to the total number of individual atoms on the product side.
When zinc metal reacts with hydrochloric acid, it leads to the production of zinc chloride and hydrogen gas. The chemical reaction follows:

<u>On reactant side:</u>
Number of zinc atoms = 1
Number of hydrogen atoms = 2
Number of chlorine atoms = 2
<u>On product side:</u>
Number of zinc atoms = 1
Number of hydrogen atoms = 2
Number of chlorine atoms = 2
Hence, the product side must be 
Physical isn't so chemical
I would always start by balancing your carbons, and then balancing the rest from there.
1. C2H5OH + O2 —> CO2 + H2O - You have two carbons on the left and one on the right. Multiply CO2 by 2.
C2H5OH + O2 —> 2CO2 + H2O
Now balance hydrogen. You have 6 on the left and 2 on the right. Multiply H2O by 3.
C2H5OH + O2 —> 2CO2 + 3H2O
Now balance oxygen. You have 3 on the left and 7 on the right. You need 4 more on the left. Don’t multiply the C2H5OH by anything because that will change the numbers of everything else too. Multiply O2 by 3 instead.
C2H5OH + 3O2 —> 2CO2 + 3H2O
Check that all atoms are now balanced, and you’re good.
2. Same process as before.
First carbons - C3H8 + O2 —> 3CO2 + H2O
Then hydrogens - C3H8 + O2 —> 3CO2 + 4H2O
Then oxygens - C3H8 + 5O2 —> 3CO2 + 4H2O
3. Same again.
Carbons) C6H12O6 + O2 —> 6CO2 + H2O
Hydrogens) C6H12O6 + O2 —> 6CO2 + 6H2O
Oxygens) C6H12O6 + 6O2 —> 6CO2 + 6H2O
4. The general reaction for a combustion reaction is a hydrocarbon reacting with oxygen to produce carbon dioxide and water.
The least reactive metal is lead, since it reacted with none of the salt solutions.