Which of the following sentences contains parallel structure?

a it is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. it is rather for us to be here dedicated to the great task remaining before us.

b four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal.

c the brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract.

d now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure.