[单选题]直流电动机无法起动，其原因可能是（）。
A.串励电动机空载运行
B.电刷磨损过短
C.通风不良
D.励磁回路断开

Harvard University, breaking with a major trend in college admissions, will eliminate its early admissions program next year, with university officials arguing that such programs put low-income and minority applicants at a distinct disadvantage in the competition to get into selective universities.
For three decades Harvard has offered a particular form of early admissions, in which students who are accepted early still have the freedom to go elsewhere. Various forms of early admissions are offered by hundreds of colleges and universities, with many requiring applicants to commit upfront(预先) to attending the university if offered early admission.
The popularity of the procedure grew significantly in the 1990s, as colleges tried to increase their competitive advantage by locking in strong candidates early. But at Harvard and many other universities officials have grown concerned about that early admissions present a major obstacle to l