If the rate-determining step limits the speed of a reaction, which statement is correct? Even if one or more fast steps occur in a reaction, the overall reaction cannot proceed any faster than the slowest step. If the first step is slow and the second step is fast, then the second step should determine the rate of the overall reaction. Increasing the concentration of a molecule that occurs in a fast step after a slow step will increase the overall rate of the reaction. In a unimolecular reaction where A decomposes to B and C, the rate of the reaction will decrease if the concentration of A is increased.

I think the correct answer would be the first option. If the rate-determining step limits the speed of a reaction, then even if one or more fast steps occur in a reaction, the overall reaction cannot proceed any faster than the slowest step. The reaction which has the slowest rate would be the rate-determining step since it is the step that takes the longest time so that no matter how fast other steps are, it is the slow step that will have a greater effect on the overall rate. It can be compared to a funnel. The rate of which a fluid would flow is determined by how big is the neck of the funnel and not the rate at which the fluid is poured into the funnel.