I also got question 13 wrong like many people. After looking at other forums I found that "in-world sales for virtual currency" they mean in the game not in the real world. So essentially the question is saying "in-game (virtual) sales for virtual currency. This is why A is wrong because it says real-life (not virtual) sales for virtual currency so D would be the correct answer. That is the only coherent way I found of understanding the question.

I notice people in the comments who have gotten it wrong give incorrect explanations or try to convince themselves why the correct answer is correct without actually understanding it. I understand it is all in good faith but it can be extremely misleading and lead to more confusion. Also it doesn't help that JY failed to explain the answers properly. If anyone from 7sage is reading this please ask him to go into more detail for each answer instead of using the "its wrong because its not right" approach. It would help us understand the question more especially for a tougher question such as this one.

I would love to join a group!

there is an implied connection between more productive use = student performance. I thought the money could have gone to other uses that would have been more productive. Even if you negate be you can still say "ok but retaining teachers is still the more productive use" but it is the answer the makes the most sense.