Guidebook writer:  I have visited hotels throughout the country and have noticed that in those built before 1930 the quality of the original carpentry work is generally superior to that in hotels built afterward.  Clearly carpenters working on hotels before 1930 typically worked with more skill, care, and effort than carpenters who have worked on hotels built subsequently.

Which of the following, if true, most seriously weakens the guidebook writer’s argument?

1. The quality of original carpentry in hotels is generally far superior to the quality of original carpentry in other structures, such as houses and stores.

2. Hotels built since 1930 can generally accommodate more guests than those built before 1930.

3. The materials available to carpenters working before 1930 were not significantly different in quality from the materials available to carpenters working after 1930.

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

5. The average length of apprenticeship for carpenters has declined significantly since 1930.

为什么答案是D呢,怎么也想不通