PrepTest 105, Section 2, Question 24
Each December 31 in Country Q, a tally(49 more words not shown)
If the statements above are(15 more words not shown)
- AIn Country Q,(11 more words not shown)
- BIn Country Q,(17 more words not shown)
- CIn Country Q,(17 more words not shown)
- DIn Country Q,(17 more words not shown)
- EIn Country Q,(19 more words not shown)
Reading the argument
The pile of coal got smaller. That tells us something about what went into it versus what came out, not how much mining or consumption changed from one year to the next.
Start with the coal left on December 31, 1990. During 1991, newly mined coal adds to that stock, and consumed coal subtracts from it. With no imports or exports, a smaller pile at the end means the country used more during 1991 than it mined during 1991.
For example, suppose the country started with 100 tons, mined 80, and consumed 110. It would finish with 70 tons. The stock fell because consumption exceeded production by 30 tons. We could change the particular numbers, but the relationship would have to stay the same.
What we’re looking for
That's my prediction for the must-be-true question: 1991 consumption exceeded 1991 mining. It could even have been a record mining year, provided consumption was greater still. The dates mark the beginning and end of one year's change in the stockpile.
Working the answers
- A
The stockpile could shrink even if mining increased. For example, the country might mine more coal than ever in 1991 and consume even more than that. Comparing 1990 mining with 1991 mining leaves out the other side of the stockpile calculation: consumption. We can't infer that mining fell merely because less coal remained.
- BCorrect
This compares the two quantities we actually need: coal consumed during 1991 and coal mined during 1991. If they were equal, the stockpile would remain the same. If mining were greater, the stockpile would grow. Since the stockpile instead shrank, consumption had to be greater.
The lack of imports and exports matters because it closes off another way to change the available supply. We aren't explaining a shrinking pile by coal leaving the country; within the given accounting, the country consumed more than it added through mining.
- C
The tally at the end of 1990 tells us what was left then, not how much was consumed throughout 1990. We cannot compare that unknown year's consumption with 1991 consumption. A shrinking stockpile in 1991 is compatible with consumption being lower than it was the previous year, but doesn't require it.
- D
This is the tempting opposite of C, and it is also unsupported. Perhaps 1991 consumption rose; perhaps it fell while mining fell even further. What matters is that 1991 consumption exceeded 1991 mining. Less coal left over doesn't necessarily mean more coal was used than in the previous year.
- E
We don't know how either year's consumption was distributed between the first and second halves. A year-end tally cannot tell us whether coal was consumed mostly in March, mostly in September, or fairly evenly. This answer also compares two different years, while the supported relationship compares consumption and mining within 1991.
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