Self-study
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. ██ ████ ████ ██████ ███ ████████████ █████ ████ ██ ███ ████ ██ █████ ████████████ ███████ █ ███ ███ ████████ ██ ████████ ████ █████ █████
Summary
At the end of each year in Country Q, there’s a count of the country’s total available coal supplies.
The total available coal supplies is the total amount of coal that has been mined throughout the country, but not yet used.
At the end of 1991 the total available coal supplies was lower than it was at the end of 1990.
Country Q has not imported or exported any coal since 1970. (So Country Q did not send any coal to another country and did not get coal from another country in 1991.)
Notable Valid Inferences
Country Q consumed more coal in 1991 than it mined in 1991. This must be true because the total available coal supplies decreased from the end of 1990 to the end of 1991. If the country had mined more coal in 1991 than it had consumed that year or an equal amount, then the total available coal supplies would have increased or stayed the same.
24.
If the statements above are █████ █████ ███ ██ ███ █████████ ████ ████ ██ ████ ██ ███ █████ ██ █████
a
In Country Q, ████ ████ ███ █████ ██ ████ ████ ███ █████ ██ █████
Could be false. We don’t know how much was mined in 1990 compared to 1991. In order to know something about the amount mined in 1990, we’d need to know how the coal supplies changed from the end of 1989 to the end of 1990.
b
In Country Q, ███ ██████ ██ ████ ████████ ██ ████ ███ ███████ ████ ███ ██████ ██ ████ █████ ██ █████
Must be true. If (B) were not true, then the coal supplies would have increased or stayed the same from 1990 to 1991. But it decreased. So more was consumed than mined in 1991.
c
In Country Q, ███ ██████ ██ ████ ████████ ██ ████ ███ ███████ ████ ███ ██████ ██ ████ ████████ ██ █████
Could be false. We don’t know how much was consumed in 1990 compared to 1991. In order to know something about the amount consumed in 1990, we’d need to know how the coal supplies changed from the end of 1989 to the end of 1990.
d
In Country Q, ███ ██████ ██ ████ ████████ ██ ████ ███ ███████ ████ ███ ██████ ██ ████ ████████ ██ █████
Could be false. We don’t know how much was consumed in 1990 compared to 1991. In order to know something about the amount consumed in 1990, we’d need to know how the coal supplies changed from the end of 1989 to the end of 1990.
e
In Country Q, ████ ████ ███ ████████ ██████ ███ █████ ████ ██ ████ ████ ███ ████████ ██████ ███ █████ ████ ██ █████
Could be false. We don’t know how much was consumed in 1990 compared to 1991. We also don’t know when, within each year, any coal consumption took place. Maybe coal consumption was equal in the first half of each year.