The Most Serious Confederate Threat to the Union Blockade Came

The most serious Confederate threat to the Union blockade came from  

A) British navy vessels on loan to the South.
B) swift blockade-running steamers.
C) the threat of mutiny from pro-southern sailors.
D) the entrance of the British and French navies as direct military combatants on behalf of the Confederacy.
E) the ironclad Merrimack (renamed the Virginia) warship, which destroyed two wooden ships of the Union navy off the Chesapeake Bay and threatened to destroy many more wooden Union warships.