That is to say, in the unfolded rectangle of the cylinder, the rectangle is divided into three small rectangles on average, and the route from A to B along the diagonal of the three rectangles is the shortest;
The radius of the bottom surface of the cylinder is 2πcm,
∴ The width of a rectangle is the circumference of the bottom of the cylinder: 2π×2π= 4cm;;
In addition, the height of the cylinder is 9 cm.
∴ One side of the small rectangle is 3 cm long;
According to Pythagorean theorem, AC = CD = DB = 5cm;;
∴ac+cd+db= 15cm;
So choose C.