How did the building of granaries indicate that Harappan civilization was economically strong?

A. It showed that the richest Harappan citizens had excess food to trade or sell.
B. It showed that Harappa could store excess food to save, sell, or trade with others.
C. It showed that Harappa could tax farmers by taking their excess crops as payment.
D. It showed that the richest Harappan citizens were capable of planning for times of drought.