"The legislative package known as the Compromise of 1850 postponed the Civil War by a decade. However, like the 1820 Missouri Compromise, the Compromise of 1850 failed to resolve the question of slavery in a meaningful way. Over the course of the 1850s, the inadequacies of both measures were
made painfully clear. 'Popular sovereignty undermined the Missouri compromise by suggesting the earlier division of the country along the thirty-sixth parallel
into free states and slave states no longer applied. Indeed, the Kansas-Nebraska Act of 1854 permitted slavery. The resulting bloodshed in Kansas, like later
incidents at Harper's Ferry, presaged the violent conflict of the Civil War."
---Library of Congress
Use the passage to answer the question
What does the author mean in saying that popular sovereignty "undermined the Missouri compromise"?
A It maintained the division of free and slave states.
B. It expanded slavery to the Northern free states.
C. It forced slaves who had escaped to be returned to a slave state
D. It reopened debate over the border between free and slave states