(20 13? As shown in the figure, the thin line connects the ball A, the two ends of the light spring connect the ball A and the ball B with the same mass, and the light with the inclination angle θ sli

(20 13? As shown in the figure, the thin line connects the ball A, the two ends of the light spring connect the ball A and the ball B with the same mass, and the light with the inclination angle θ slides obliquely. A, let the mass of both balls be m, and before the thin thread is burnt out, take ball B as the research object, and calculate the spring force F=mgsinθ according to the equilibrium condition.

At the moment when the thin wire burns out, the elastic force of the spring remains unchanged, so the force on the B ball remains unchanged and the instantaneous acceleration is zero.

At this time, the resultant force on the ball A is F+mgsinθ=2mgsinθ, and the direction is downward along the inclined plane.

According to Newton's second law, the acceleration of ball A is aA=2mgsinθm=2gsinθ, and the direction is downward along the inclined plane, so A is wrong and B is correct;

C, because A has acceleration, ABC is regarded as a whole. According to the decomposition of motion, the whole has acceleration in both vertical and horizontal directions. According to Newton's second law, the vertical acceleration of the whole body is downward, so the supporting force of the ground against C is less than the sum of the gravities of A, B and C, that is, C? The pressure on the ground is also less than the sum of gravity of A, B and C, so C is wrong;

D. according to the analysis of option c, the whole body has horizontal acceleration to the left. According to Newton's second law, the direction of static friction of the ground against C is horizontal to the left, so D is wrong.

So choose: B.