Why was the admission of Maine into the Union at the same time as Missouri a significant provision of the Missouri Compromise?

A) The Constitution requires states be admitted as pairs as a part of the annexation process.
B) Northern states wanted a gateway to Canada since southern states were gaining access to the Mississippi River.
C) Both Maine and Missouri were to serve as substantial military posts in the ongoing wars with Native Americans.
D) The slave-holding states and the free states of the Union wanted to maintain a balance of power in the representation in the U.S. Congress.