Which definition of the term "border state" is correct?

a) a slave state that seceded during the civil war.
b) a free state that joined the confederacy
c) a slave state that stayed in the union
d) a free state that lies next to a slave state

because its important to learn about other countries and how high tech they are compared to us. also its good to learn about other cultures and how ours differs and in common also we need to learn about companies that we still buy since they ship it here.