Which statement best describes the contributions of Frederick Douglass during the Civil War?
A. Douglass was an abolitionist who served the Union in the 54th Massachusetts Regiment.
B. Douglass was an abolitionist who formed the flying squadron known as the Tuskegee Airmen.
C. Douglass was an abolitionist who encouraged the Union to allow African Americans to enlist.
D. Douglass was an abolitionist who helped enslaved people escape through the Underground Railroad.