The African diary of Joao Infante, a 15th century Portuguese explorer

Confused BTW B and D. As B also give evidence to show the following.

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Optio B not necessarily gives evidence as the account of two other authors can be wrong as well as right so it does not support the conclusion always

A. the writings of almost all Portuguese explorers have been discovered to be extremely accurate.
- This statement about the accuracy of Portuguese explorers' writings is not relevant to the argument's conclusion that there is no reason to believe Infante's entries were incorrect based on the absence of the horn in fossils. It doesn't provide a direct reason for why the absence of the horn in fossils doesn't necessarily mean Infante's entries were incorrect.

B. later works by British and Spanish explorers written between the 17th and 19th centuries also describe the Malagasy Hippopotamus as a horned animal.
- This statement suggests that other explorers' writings also describe the Malagasy Hippopotamus as a horned animal. While it provides some historical corroboration, it doesn't directly address the issue of the absence of the horn in the recently discovered fossils. Therefore, it doesn't complete the argument logically.

C. another species of African hippopotamus that currently exists also has keratin deposits which resemble a horn on its snout.
- This option introduces information about another species of African hippopotamus. While it touches on the idea that the presence of a horn-like structure may not necessarily mean the Malagasy Hippopotamus had one, it doesn't directly address the absence of the horn in the fossils discovered last year. Therefore, it doesn't provide a direct reason for why Infante's entries might still be accurate despite the absence of the horn.

D. animal horns are composed only of the protein keratin and do not fossilize.
- This option provides a logical explanation for why the absence of the horn in the fossils does not necessarily mean that Infante's entries were incorrect. If animal horns do not fossilize and the horn was made of keratin, it could have existed on the Malagasy Hippopotamus without leaving fossil evidence. This directly supports the argument's conclusion and completes it logically.

E. Infante is believed to have made his diary entries as and when he witnessed events, to preserve the accuracy of his writings.
- This statement about the timing of Infante's diary entries doesn't directly address the issue of the absence of the horn in the fossils. While it speaks to Infante's credibility, it doesn't provide a reason for why the absence of the horn in the fossils might not be a valid reason to doubt his entries. Therefore, it doesn't complete the argument logically.

In summary, option (D) is the most logical completion because it directly addresses the issue of fossilization and provides a reason for why the absence of the horn in the fossils doesn't necessarily mean that Infante's entries were incorrect.