Under the bimetallic system, when did the U.S. experience a depletion

Under the bimetallic system, when did the U.S. experience a depletion in its reserve species?

(a) When imports exceeded exports
(b) When exports exceeded imports
(c) When the U.S. closed its borders to all international trade
(d) When the trading partners of the U.S. refused all U.S. imports

ANSWER

(a)