Not all profs make it so you can get As and Bs. I just graduated 2 years ago, and Everyone was studying like crazy to pass the classes (well except one person who had photographic memory). But it also depends on the area of study and the school. Sometimes in grad school getting/keeping stipends/workstudy/grants depends on you getting the best grades in your section, which can cause competition between students. So it really depends on how the department is set up. And also if you are working in a profesor's lab, you are at their mercy for what happens to you.