Why Did George Washington Eventually Allow African-Americans to Serve in the Continental

Why did George Washington eventually allow African-Americans to serve in the Continental army?

A) Southern colonies stopped their opposition.
B) Many northern colonies immediately abolished slavery.
C) He had freed all of his slaves during the second year of the war.
D) The British started offering freedom to slaves who signed up to fight for their army.
E) He became convinced that African-Americans could fire a musket.