The main reason for the wall's construction had always been defense. Since China's first dynasty, the empire faced a near-constant threat of attack from the nomadic tribes to the north. This included those famed Mongols as well as lesser-known groups like the Xiongnu and the Göktürks.
The answer is A) by offering incentives to families who only have one child
So that people have to pay for their effort. As you know everyone in the community needs to earn money somehow in order to live.
Also used for copy protecting in short term to honor them as well
