Sampling in Official GMAT CR Questions

Ok. Let me use some numbers here.

Suppose there are 1000 hotels that were created before 1930 and 1000 hotels that were created after 1930.

For the hotels created before 1930,

A1:Quality of 300 hotels = 100
A2: Quality of 400 hotels = 200
A3: Quality of 300 hotels = 300

Average quality = 200

For the hotels created after 1930,

B1: Quality of 300 hotels = 100
B2: Quality of 400 hotels = 200
B3: Quality of 300 hotels = 300

Average quality = 200

So, average quality for hotels from both period is same.

Now, let's bring option D into the picture.

The better the quality of original carpentry in a building, the less likely that building is to fall into disuse and be demolished

Now, tell me of hotels created before 1930, which hotels are more likely to exist now. A3? Right. After that? A2?

So, now let's suppose currently we have the following pre-1930 hotels left

A1: 150 (half of them got demolished)
A2: 300 (25% of them got demolished)
A3: 300 (None of them got demolished)

Average quality now = 2.2

For hotels built after 1930

B1: 240 (20% of them got demolished) (Since these hotels are built after 1930 and are newer than those built before 1930, the ratio of hotels that got demolished will be lower)
B2: 360 (10% of them got demolished)
B3: 300 (None of them got demolished)

Average quality now: 2.06

So, even though initial quality is same, because of option D, you see final quality of pre-1930 better than post-1930 hotels.

Just to emphasize, I did not chose these number to make option D correct; I chose them out of logic.

Let me know if it addresses your doubts.

Thanks,
Chiranjeev