A rectangular piece of paper whose length is times the width has area A. The paper is divided into three equal sections along the opposite lengths, and then a dotted line is drawn from the first divider to the second divider on the opposite side as shown. The paper is then folded flat along this dotted line to create a new shape with area B. What is the ratio B:A? ( )
1:2
3:5
2:3
3:4
4:5