Answer:
5 1 2 4and 3 this is correct way
The B. Ca-F pair has the greatest electronegativity difference.
Electronegativities <em>increase from the lower left to the upper right</em> in the Periodic Table.
Ca is closest to the lower left, and F is in the upper right, so the Ca-F bond has the greatest electronegativity difference.
The answer is catalyst.
A catalyst is characterized by acceleration the rate of a chemical reaction but not modifiy the equilibrium, which is what k represents (the constant of equilibrium of the reaction).
The catalyst manages that the reaction goes through a different pathway, with a lower activation energy, so the transition compound is reached faster and the whole reaction goes faster, but the final species and their equilibriium concentrations do not change.