Hi mukherjee.tanuj3,

This inference question does not offer us much information to work with, so we have to be very careful about what we KNOW and what we DON'T KNOW.

The Facts:
-New technology is being used in Tribnia's coal industry (specifically in its coal mines).
-Coal output PER MINER, for all of Tribnia's coal industry, is double what it was 5 years ago.
-NO new mines have opened.

What we DON'T KNOW:
-How many mines there are.
-How many miners there are.
-How much mining any individual miner does.

We can use the above information to assess the answer choices:

Answers C, D and E can be quickly eliminated:
C because it involves a hypothetical that does not match the restrictions in the prompt.
D because it involves the output of any one individual miner, which we know nothing about.
E because it involves cost of coal, which is irrelevant.

Between A and B....
A says if the number of miners stays the same, then the total output has doubled. This MATCHES the above facts since coal output per miner has doubled. If the number of miners is the same, then the "doubling" would double the output.

B says any mine that had an increase had an increased output per miner. That is NOT necessarily true. If the mine hired additional miners, then the total output could have increased but the average per miner (for THAT mine) could have decreased.

Final Answer: A

GMAT assassins aren't born, they're made,
Rich