Seen from the side, when water touches the sphere, there is still 2 cm above the square. This is a right triangle. Find the hypotenuse (hypotenuse is the radius of the ball), the square of R = the square of (R-2)+the square of 4, and the radius of the ball is 5 cm, so the answer is A.

Your classmate's words probably tell you why one side of this right triangle is equal to 4 (that is, half the length of a cube).

Because the ball is placed on the top of the cube (very stable, no external force), the center of the ball must be in a straight line with the center of the cube. So when you draw a picture, the right-angled side of the triangle passes through the center of the cube, so the other right-angled side is half the length of the side.