Each December 31 in Country Q, a tally is made of the country's total available coal supplies—that is, the total amou...

Emil-Kunkin on June 5, 2022

Hi Bsasserson,

We are told that a country who neither exports nor imports coal keeps a tally of their coal stock. The only way that increases is mining, and the only decrease comes from consumption. Thus, if the stock increases, the country must have mined more than it consumed, and if the stock decreases, they must have used more than they mined.

We are told that the stock of coal was lower in 91 than it was in 90, so it must be the case that the country used more coal than it mined, which is an exact match for B.