题干
—Students are supposed _______.
—I agree with you.
A：study hard
B：studying hard
C：to studying hard
D：to study hard
答案(点此获取答案解析）
D
—Students are supposed _______.
—I agree with you.
A：study hard
B：studying hard
C：to studying hard
D：to study hard
下降到虚线位置，但圆环仍保持在原来位置不动。则在这一过程中，环对杆的摩擦力F1和环对杆的压力F
2的变化情况是 
