During World War I, Germany Used a "Big Bertha" Cannon

During World War I, Germany used a "Big Bertha" cannon to hurl shells into Paris 30 miles away. This gun also had a very long barrel. What was the reason for using a long barrel in these guns?

A) to reduce the force exerted on the bullet due to the expanding gases from the gunpowder
B) to reduce frictional losses
C) to increase the force exerted on the bullet due to the expanding gases from the gunpowder
D) to allow the force of the expanding gases from the gunpowder to act for a longer time
E) to exert a larger force on the shells