A、Those that are involved in a specific field. B、Those that are very difficult and urgent. C、Those that need cooperation of two

We continue our discussion of plagiarism(剽窃). Last week, we said colleges and universities in the United States define plagiarism as representing another person’s work as your own. It’s considered a kind of cheating. Professors at American colleges have tried many ways to stop students from plagiarism.(16)Some use online detection services. They also may discuss plagiarism with their students at the start of every term. Some require their students to turn in early versions of term papers, research papers and essays they are writing. This makes it more difficult for students to buy papers from companies that some call "plagiarism mills" or "essay mills." A recent report described such businesses. They sell newly-written papers on many subjects.(17)The cost depends on the difficulty of the subject and how soon the paper is needed. The cost could be from $ 20 to $ 40 a page. Such companies say their writers have advanced degrees, and will target the papers to any educational level. Investigators say the writers may be working in countries like India, Nigeria or Indonesia and are poorly paid.(18)Most of these companies say their work should only be used as models and should not be turned in as a finished work. But students do it anyway. University of Notre Dame anthropology professor Susan Blum wrote about this in a new book, My Word! Plagiarism and College Culture, in which(19)she writes that academic cheating is a result of communication failure between students and professors. And she says international students must be sure they know the rules of the college they are attending.
16. Which of the following is one of the measures to stop plagiarism?
17. What essays may "essay mills" charge the most?
18. What do most essay companies claim?
19. What leads to academic cheating according to Prof. Susan Blum?