Noting the success of DVDs in the marketplace, the newly opened video store Movie Tapes, which sells specialty videocassettes, began an ambitious publicity campaign to promote the sale of their tapes over more technologically advanced DVDs. After six months in business, Movie Tapes found that its sales had doubled - in fact, it had sold twice as many copies of movies on tape as all the other stores in town had sold on DVD of the same movies. Clearly, the publicity campaign was responsible for Movie Tapes' video cassettes selling better than DVDs.

Which of the following, if true, casts the most serious doubt on the conclusion drawn above?

A) DVDs include special features that enhance the viewing experience.
B) The local school system uses only VCRs in the classroom.
C) Only a very small number of the movies the store sells are available on DVD.
D) A month before the campaign began, the public library started buying videocassettes to lend.
E) Neither the price of DVDs nor that of videocassettes has risen in two years.

I have a question about the answer: although i understand that C offers an alternative explanation as to why the sales went up, i don't understand how it covers the part about "it had sold twice as many copies of movies on tape as all the other stores in town had sold on DVD of the same movies."

Thank you for your help!