The correct answer is: C. Either I or II is implicit.
The statement “A good book, even if costly, is sold” does not provide enough information to determine whether or not assumption I (Some books are better than others) or assumption II (Most of the books are costly) is true. It is possible that a good book is sold because it is better than other books, or it is possible that a good book is sold despite being costly. Therefore, either assumption I or assumption II could be true, but we cannot say for certain which one is true.
Here is a possible scenario in which assumption I is true: A new book is released that is considered to be a masterpiece. It is so good that people are willing to pay a high price for it, even though there are other books available that are much cheaper. In this case, the assumption that some books are better than others is true, because the new book is considered to be better than other books on the market.
Here is a possible scenario in which assumption II is true: A book is released that is considered to be a must-read for anyone interested in a particular topic. It is so popular that people are willing to pay a high price for it, even though there are other books available that are much cheaper. In this case, the assumption that most books are costly is true, because the book is in high demand and there is a limited supply.
However, it is also possible that neither assumption I nor assumption II is true. For example, a book might be sold because it is the only book available on a particular topic, or because it is the only book that is written in a particular language. In these cases, the assumption that some books are better than others, or the assumption that most books are costly, is not true.
Therefore, the correct answer is: C. Either I or II is implicit.