The following statements about the Fall Line are true:
A. It makes the northern edge of the Piedmont Region - True. The Fall Line generally marks the boundary between the Piedmont and Coastal Plain regions in the eastern United States.
B. Rivers widen and move more slowly south of it - True. South of the Fall Line, rivers tend to be broader and slower-moving compared to the faster-flowing rivers found to the north.
C. Many cities have developed along it - True. The Fall Line has historically been a significant area for settlement and economic development, and many cities, such as Richmond, Virginia, and Columbia, South Carolina, have developed along this line due to the presence of waterfalls and rapids.
D. It contains large numbers of swamps - False. The area of the Fall Line itself does not typically contain large numbers of swamps. Swamps are more commonly found in the Coastal Plain region, particularly south of the Fall Line.
So, the correct true statements about the Fall Line are A, B, and C.