In Bertrand and Mullainathan (2004), what is the key aspect of the experiment that allows the researchers to isolate discrimination as the source of a given disparity

a. The researchers repeated the experiment multiple times, showing the disparity is externally valid
b. A large sample size that sent applications to both Chicago and Boston
c. They designed the applications so that Black applicants had more experience and better education relative to white applicants
d. Randomization of names to applications