The Reason Grooves Are Rifled into the Bore of a Gun

The reason grooves are rifled into the bore of a gun is so that a:

A) bullet will be reduced in size before it exits the gun.
B) bullet will be made to spin and hence have a true and accurate course on leaving the barrel.
C) ballistics examiner will have an easier time identifying evidence.
D) manufacturer can put its unique mark on its product.
E) bullet moving through it will have unique striations.