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?
(A) In Country Q, more coal was mined in 1990 than was mined in 1991. - WRONG. Both side possibilities exist.
(B) In Country Q, the amount of coal consumed in 1991 was greater than the amount of coal mined in 1991. - CORRECT.
(C) In Country Q, the amount of coal consumed in 1990 was greater than the amount of coal consumed in 1991. - WRONG. Again both side possibilities exist.
(D) In Country Q, the amount of coal consumed in 1991 was greater than the amount of coal consumed in 1990. - WRONG. 100 in 1990 and 1000 in 1991. Still, the left amount of coal considerably can be lower in 1991 than what it was in 1990. However, the reverse is also possible - 1000 in 1990 and 100 in 1991. The latter case is also possible because mined coal could have been so high in 1990 that higher consumption in 1990 relative to 1991 was possible.
(E) In Country Q, more coal was consumed during the first half of 1991 than was consumed during the first half of 1990. - WRONG. We can't be sure.
A difficult one.
The best part is that it asked for a 'must be true' not 'sufficient', otherwise it would have left opened so many choices.
OR
May be choices where they would have been far more vague.
So what do we do when after consumption of mined coal the amount of coal left in 1991 was considerably lower than it had been in 1990.
Possibly, no coal is mined in 1991 with leftover of 1990 is used. But is it necessary such that no other possibility exists. No!! - there can be other possibilities still.
What makes B difficult is that it compares amount of coal consumed in 1991 to the amount of coal mined in 1991, which leads one to believe that such a comparison is irrelevant or at best not possible.
Consider this:
1990:
Mined - 1000, Consumed - 900, Left - 100
1991:
Mined - 1000, Consumed - 990, Left -
110 (Not possible)
Mined - 100, Consumed - 90, Left -
110 (Not possible)
Mined - 1000, Consumed - 1090, Left -
10 (Possible)
Mined - 0, Consumed - 50, Left -
50 (Possible)
Mined - 0, Consumed - 10, Left -
90 (Possible)
Last one makes things difficult though.
So, all in all, the consumption of coal has to be more than mined coal in 1991.
Answer B.
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