This involves the interaction between three objects. In A, the blackboard is not in contact with the magnetic block, so there is no friction relationship. The paper in B is balanced, so it won't fall off because of friction. The paper is subjected to the upward friction of the blackboard, the downward friction of the magnetic block and its own gravity, so C is wrong; Answer d

I'm science teacher Wang, 2 1 Jinhe Building, next to Zhejiang No.1 Hospital.