In the mathematical equation showing that volume (V) divided by temperature (T) remains equal to a constant (k), volume and temperature are proportional

Directly  

Explanation:

If V increases, then T must also increase to keep the same ratio.

If V decreases, then T must also decrease to keep the same ratio.

V and T are directly proportional, because a change in one always causes a change in the other and the changes are in the same ratio. The constant k is called the proportionality constant.

elements make up everything, and they combine to make compounds.

the answer is c

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.