What holds for sequential allostery?

14. Which of the following is a defining characteristic of an allosteric protein demonstrating sequential kinetics?
a. The enzyme interconverts between the T and R states in the absence of substrate.
b. In the enzyme complex, monomeric units might be in a mix of R and T states.
c. The binding of substrate increases the affinity of the complex for additional substrate molecules
d. Once a substrate binds to the R state, all of the monomeric units are "locked" into the R state.
e. The substrate binds only to the monomers previously converted to the R state.

I'm not sure if the the correct answer is b or c (I do not think the other suggestions are correct). It is the induced fit protocol vs completely conformational change concerning substrate binding I am not certain about. See link: (http://en.wikipedia.org/wiki/Allosteric_regulation)