The motion of a simple pendulum is simple harmonic motion, and simple harmonic motion requires that the restoring force is inversely proportional to the displacement: F=-KX Because the displacement is changing, it means that the restoring force F is also changing;

Ignoring the air resistance, the force on the pendulum ball is analyzed. The pendulum ball is subjected to gravity (vertically downward) and cycloidal tension (cycloidal contraction direction). Because the trajectory of the pendulum ball is in the tangent direction of the arc, a coordinate system is established based on the tangent direction to decompose gravity.

X-axis: Gx is the tangential direction along the pendulum motion.

Y axis: the tangent of Gy motion is vertical.

X-axis tangent direction equation, Gx=ma

At this time, Gx is the resultant force of the pendulum ball, pointing to the equilibrium position, and the component of gravity decreases continuously along the X axis during the movement until the pendulum ball moves to the horizontal position, and the component of gravity is 0;

To sum up, in fact, the restoring force of a simple pendulum is provided by the gravity component, named from the perspective of effect; So BC is right and AD is wrong;

So choose BC.