Critical Reasoning

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!

the fact that very less movies are available in DVD format gives us reason to believe that people in this utopian land have no other choice but to buy movies only in that special cassette and thats the reason the sale of this special cassette might have doubled

thank you for your reply
i am still confused about how the answer choice supports why the sales of videos that are also available on DVD went up as well?

thank you!

I do not agree with the official answer here... infact the best one is D.

the result - "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" --- so the relative number of movies won't make any difference.

What is the source ?

This was from a free practice test on Kaplan.com