Each December 31 in Country Q, a tally is made of the country' s total available coal supplies -- that is, the total amount of coal that has been mined throughout the country but not consumed. In 1991 that amount was considerably lower than it had been in 1990. Furthermore, Country Q has not imported or exported coal since 1970. If the statements above are true, which one of the following must also be true on the basis of them? ['In Country Q, the amount of coal consumed in 1990 was greater than the amount of coal consumed in 1991.', 'In Country Q, more coal was consumed during the first half of 1991 than was consumed during the first half of 1990.', 'In Country Q, the amount of coal consumed in 1991 was greater than the amount of coal mined in 1991.', 'In Country Q, more coal was mined in 1990 than was mined in 1991.']