This is a must be true question. Therefore, you need to be able to extract statement(s) from the stimulus that can prove your answer choice/conclusion.
Here is why answer choice A is correct:
Coal output per miner is double what it was five years ago.
Let's say there were 2 coal miners five years ago, and their output was 5 units of output per miner or a combined total of 10 units. It says now that the coal output per miner is double what it was five years ago. Ok, so now each of the two coal miners produce 10 units of output per miner for a combined total of 20 units (10+10 from 5+5). Keep in mind that I have brought in no new or additional assumptions/pieces of information. All I did was use example numbers (5 and 10) to explain the stimulus in less abstract terms. What does answer choice A say?
If the number of miners working in Tribnian coal mines has remained constant in the past 5 years, Tribnia's total coal production has doubled in that period of time. As you can see from the example above, the output is now 20 compared to the output 5 years ago of 10. The output has doubled, but only because the number of workers stayed the same. If the number of workers increased by 1, the output per miner would now be 10 per miner for a combined output of 30 (10 units of output x 3 people). Is this double the output 5 years ago? No, it is triple, because the number of miners have gone up. Here, I have not added any new assumptions or information -- I have extrapolated this entire conclusion from the stimulus.
I understand why answer choice C is attractive. The answer is very likely to be true. If more mines opened, there would be more coal production and potentially, more coal output per miner, but this is not NECESSARILY true. You had to ASSUME that more mines would open. You never have to assume on a MUST BE TRUE. It must be true given the elements of information already provided to you in the stimulus. Prove your answer from the stimulus in logical steps -- do not add any assumptions of your own.
Hope this helps...
