During the revolution of the earth, the direct point of the sun moves regularly between the Tropic of Cancer and Tropic of Cancer, which leads to the change of seasons and the change of solar altitude angle.

Combined with the meaning of the question, "it is found that the shadow of the flagpole is shorter every day, and the length of the shadow is obviously shorter than the length of the flagpole", which shows that the height angle of the sun is getting bigger and bigger, and the shadow will become shorter and shorter.

Combined with the chart, the earth should move from the equator to the Tropic of Cancer during the operation from A to B.

So choose: a.