Answer: 
is a reactant; it is present before the reaction occurs.
Explanation:
In a chemical reaction the chemical formulas written before the arrow are described as reactants as they react together to form products which are written after the arrow.
Thus and HCl are reactants here whereas 
, 
and 
are products.
Answer:
Oxygen is the most contain in earth but you asked in a,b,c and d. So I'll choice A. aluminium.
Explanation:
What benzene is
Benzene is a chemical that is a colorless or light yellow liquid at room temperature. It has a sweet odor and is highly flammable.
Benzene evaporates into the air very quickly. Its vapor is heavier than air and may sink into low-lying areas.
Benzene dissolves only slightly in water and will float on top of water.
