What made Whistler’s work more valuable?

Câu hỏi: Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.
The Art World
One of the major problems in the art world is how to distinguish and promote an artist. In effect, a market must be created for an artist to be successful. The practice of signing and numbering individual prints was introduced by James Abbott McNeill Whistler, the nineteenth-century artist best known for the painting of his mother, called "Arrangement in Grey and Black", but known to most of us as "Whistler's Mother". Whistler's brother-in-law, Sir Francis Seymour Haden, a less well-known artist, had speculated that collectors might find prints more attractive if they knew that there were only a limited number of copies produced. By signing the work in pencil, an artist could guarantee and personalize each print.
As soon as Whistler and Haden began the practice of signing and numbering their prints, their work began to increase in value. When other artists noticed that the signed prints commanded higher prices, they began copying the procedure.
Although most prints are signed on the right-hand side in the margin below the image, the placement of the signature is a matter of personal choice. Indeed, prints have been signed within image, in any of the margins, or even on the reverse side of the'print. Wherever the artist elects to sign it, a signed print is still valued above an unsigned one, even in the same edition.
What made Whistler's work more valuable?
A. His fame as an artist.
B. His painting of his mother.
C. His signature on the prints.
D. His brother-in-law's prints.