How did the one-child policy result in large numbers of undocumented Chinese?

How did the one-child policy result in large numbers of undocumented Chinese?

The Chinese government could not keep up with the vast amount of paperwork required to document children.
Many Chinese parents protested the one-child policy by refusing to document the children in their household.
Some mothers thought their children would have a better chance of leaving China if they remained undocumented.
After their first child, many mothers hid any other children they had, thereby causing them to be undocumented.