Answer: We are evolving to the environment, and the changes around us. And those that don't die
Explanation:
They put pressure on us to adapt in order to survive the environment we are in and reproduce. It is selection pressure that drives natural selection ('survival of the fittest') and it is how we evolved into the species we are today. Genetic studies have demonstrated that humans are still evolving.
Not (A) because a molcule with pH 7 is neutral eg, water
Not (B) because a molecule with pH >7 is basic/alkali which is the opposite of acid.
This leaves C and D.
Lets take one of the most commonly known acids, hydrochloric acid which has the formula HCl.
When acids are added to water they split up (dissociate).
HCl cannot split to give hydroxide ions but can give hydrogen atoms (which are actually hydrogen ions > H+)
so the answer is D
Cellular junctions: by connecting cells to their neighbors
Receptors: allowing cells to respond to chemical messages