【单选题】

Students who entered lotteries and won spots in New YorkCity charter schools performed better on state exams than students who entered the same lotteries but did not secure charter school seats, according to a study by a Stanford University economist being released recently.

Charter schools, which are privately run but publicly financed, have been faring well on standardized tests in recent years.But skeptics have discounted their success by accusing them of "creaming" the best students, saying that the most motivated students and engaged parents are the ones who apply for the spots.
The study’s methodology (研究方法) addresses that issue by comparing charter school students with students of traditional schools who applied for charter spots but did not get them. Most of the city’s 99 charter schools admit students by lottery.
The report is part of a multi-year study examining the performance of charter schools in New YorkCity byCaroline M. Hoxby, a Stanford economist who has written extensively about her research on charter schools and vouchers.
Ms. Hoxby found that students who attended a charter school from kindergarten to eighth grade would nearly match the performance of their peers in rich suburban communities on state math exams by the time they entered high school, a phenomenon she characterizes as closing the "Harlem-Scarsdale" achievement gap. The results are somewhat less striking inEnglish, where students closed 66 percent of the gap, according to the study.
By the third grade, according to the study, the average charter school student was 5.3 points ahead on state exams inEnglish compared with students who were not admitted to the charter schools. In math, the students were 5.8 points aheaD、Most tests are scored on a scale of roughly 475 to 800.
Ms. Hoxby did not reach any conclusions about what practices at the schools caused the jump, but she noted that many charter schools had extended school days and school years, many required students to attend classes on Saturdays and most paid teachers based on their performance and responsibility, rather than the traditional teachers’ union salary scales.
Mayor Michael R.Bloomberg and the schools chancellor, Joel I. Klein, have embraced charter schools as a key to their effort to overhaul (彻底改革) the city’s school system. Mr. Klein has made an effort to recruit charter school operators that have been successful in other parts of the country to open schools throughout the city, particularly in the SouthBronx,CentralBrooklyn and Harlem. There are approximately 30,000 students in charter schools in the city, and another 40,000 students on waiting lists to be admitted to those schools.
People who suspect charter schools’ achievement attribute these schools’ success to their ______.A．prolonging students’ school time
B．bribing educational testing system
C．skimming off the best students
D．burdening students with heavy assignments