The Northerners
worried that low labor costs of the South would hurt the North's economic power.
The Northerners have always wanted to block the spread of slavery and were concerned that an extra slave state would give the South a political advantage.
However, the Southerners wanted the new states to be free to allow slavery if they prefers to.
Therefore, the Option D. is correct because the Northerners
worried that the low labor costs of the South would hurt the North's economic power.
Read more about this here
brainly.com/question/1429340