Passage 5
Criticism of research lays a significant foundation for future investigative work, but when students begin their own projects, they are likely to that the standards of validity in fieldwork are considerably more rigorous than the standards for most library research. When students are faced with the concrete problem of proof by field demonstration, they usually discover that many of the "important relationships" they may have criticized other researchers for failing to demonstrate are very elusive indeed. They will find, if they submit an outline or questionnaire to their classmates for criticism, that other students make comments similar to some they themselves may have made in discussing previously published research. For example, student researchers are likely to begin with a general question but find themselves forced to narrow its focus. They may learn that question whose meanings seem perfectly obvious to them are not clearly understood by others, or that question which seem entirely objective to them appear to be highly biased to someone else. They usually find that the formulation of good research questions is a much more subtle and frustrating task than is generally believed by these who have not actually attempted it.
According to the passage, student researchers may have to change their research projects because（）
A．their budgets are too high
B．their original questions are too broad
C．their teachers do not give adequate advice
D．their time is very limited
