Answer:
Polluted ground water.
Explanation:
The rocks through which water flows into the ground sieves most of the contaminants whose molecules are larger than most bacteria. Such molecules include but are not limited oil molecules. Therefore as it descends to the lower levels of the soil profile, water is subjected to continuous filtration by the layers and it is the least susceptible to contamination by oil spills.
B is the answer. For an equation to be balanced there must be the same amount of each element on the two sides. The only one where this is the case is B.
Explanation:
Mole ratio of Oxygen to Hydrogen gas = 1 : 2.
If we use 3.0 moles of oxygen gas, we would need 3.0 * 2 = 6.0 mol of hydrogen gas.
However we only have 4.2 mol of hydrogen. Therefore hydrogen is limiting and oxygen is in excess. (B)
Answer: 
Explanation:
For formation of a neutral ionic compound, the charges on cation and anion must be balanced. The cation is formed by loss of electrons by metals and anions are formed by gain of electrons by non metals.
Here iron is having an oxidation state of +3 called as 
cation and oxide 
is an anion with oxidation state of -2. Thus they combine and their oxidation states are exchanged and written in simplest whole number ratios to give neutral 
.
According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
