考查细节推断。根据题干信息可以将答案锁定在第三段Primarily, there's the convenience promised by courses on the Net: you can do the work, as they say, in your pajamas (睡衣).这里提到网络课堂更为便利：你可以穿着睡衣上课。这里的细节推断就是说无论何时何地你都可以进行学习。04.What accounts for the high drop-out rates for online students?
AThere is no mechanism to ensure that they make the required effort.
BThere is no strict control over the academic standards of the courses.
CThe evaluation system used by online universities is inherently weak.
DLack of classroom interaction reduces the effectiveness of instruction.