Since few of his students used painting techniques that differed from Larocque’s, and Nightbird “was painted in a style indistinguishable from that of Larocque,” (A) strengthens the conclusion that Nightbird was painted by one of Larocque’s students.

The fact that Larocque never signed any of his paintings does not weaken the conclusion that Nightbird was painted by one of his students. Perhaps his students never signed their paintings either.

The key reason for concluding that Nightbird was painted by a student of Larocque was the orpiment, which paintings attributed to Larocque don’t have. But if none of his students’ paintings contain orpiment either, why should we still conclude that it was painted by one of them?

The importance of Larocque’s students has nothing to do with the conclusion that one of his students painted Nightbird. As far as we know, an unimportant artist could have painted Nightbird just as easily as an important one.

Just because orpiment was more popular after Larocque’s death doesn’t mean that it didn’t exist or that he didn’t use it before his death. It also doesn’t mean that his students, many of whom presumably outlived him, could not also have used orpiment after well after his death.