What analogy does the writer samuel johnson create in this excerpt from rasselas?

i remembered that my father had obliged me to the improvement of my stock, not by a promise, which i ought not to violate, but by a penalty, which i was at liberty to incur; and therefore determined to gratify my predominant desire, and, by drinking at the fountain of knowledge, to quench the thirst of curiosity.

a. stock is to promise as penalty is to liberty
b. stock is to penalty as promise is to liberty
c. fountain is to thirst as knowledge is to curiosity
d. thirst is to knowledge as fountain is to curiosity