Why does a higher concentration increase the rate of reaction?

A higher concentration will increase rate of reaction because in a conc substance the particles are more clustered together and this causes more effective collisions thus increasing the rate of reaction.

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).

the answer is b. dilution