For statement (1), we know that Thursday sold 8 books, leaving 82 total remaining, and that Saturday and Friday must be above 8. It is easy to create a scenario here where Friday is over 11 and one where it is not, making Saturday's sale as large as necessary. Say, Friday is only 9, and we can arbitrarily pick numbers for the other five days to total up to 90. We may also say Friday is 12, and once again arbitrarily pick numbers for the other five days to total up to 90. We just don't know. (1) is insufficient.
For statement (2), we know that 38 books can be subtracted from the original 90 (leaving 52), and all other values must be less than 38. Obviously if we choose the other five days to be low (1, 2, 3, 4, 5), Friday will sell eleven or more books. But can we manufacture a situation where Friday will sell eleven or less books? Let's maximize the other five days: 10, 9, 8, 7, and 6 (since they must be under Friday) . This is a total of 40 books, which subtracted from 52, is 12. This is Friday's worst-case scenario, and so this is sufficient information to answer the question.
The answer is B.
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