The L-shaped board P (with smooth upper surface) is placed on the fixed inclined plane. One end of the light spring is fixed on the board, and the other end is connected with the sliding block Q placed on the upper surface of the board, as shown in the figure. If P and Q slide down the inclined plane at a constant speed, regardless of the air resistance, the number of forces on the board P is () A. 3B. 4C. 5D. 6

The L-shaped board P (with smooth upper surface) is placed on the fixed inclined plane. One end of the light spring is fixed on the board, and the other end is connected with the sliding block Q placed on the upper surface of the board, as shown in the figure. If P and Q slide down the inclined plane at a constant speed, regardless of the air resistance, the number of forces on the board P is () A. 3B. 4C. 5D. 6

P. According to Newton's third law, the object Q must have pressure on the object P, and the spring must also have downward force on P. therefore, the board P is subject to gravity, the supporting force of the inclined plane, the friction force of the inclined plane, the pressure of Q and the downward force of the spring along the inclined plane Therefore, option C is correct