"By the rude bridge that arched the flood, Their flag to April's breeze unfurled, Here once the embattled farmers stood, And fired the shot heard round the world."

These lines are from a hymn written by Ralph Waldo Emerson in 1837. To what do the lines refer?

A. The beginning of the revolutionary war.

B. Washington's crossing of The Delaware River before the battle of Trenton.

C. The battle at the Boston Harbor.

D. The end of the revolutionary war.