Fill in the blank (_) in the following sentence from the following choices.
Sentence: The librarian wanted to put the books on the shelves but they weren't able to hold them because the _ were too flimsy.

Choice 1: The librarian wanted to put the books on the shelves but they weren't able to hold them because the shelves were too flimsy.
Choice 2: The librarian wanted to put the books on the shelves but they weren't able to hold them because the books were too flimsy.
Which of the choices is correct? Respond with only "1" or "2" and no additional text.
1
2.