B, because the absolute value of the slope of the E-x image of the object in S 1 ~ S2 is unchanged, the tensile force on the object is unchanged. If the tension is equal to the gravity on the object, the object moves in a straight line at a uniform speed. Therefore, B is correct.
C, because the object in S 1 ~ S2 has the same tension and acceleration, it is impossible for the object to move in a straight line with variable acceleration, so C is wrong.
D. If the pulling force of the rope on the object is less than the gravity of the object in the range of 0 ~ S2, the object will move downward with acceleration, so the kinetic energy of the object will increase continuously, so D is correct.
So choose b and d.