For what purpose was the phrase "one if by land, two if by sea" used

It was to know how many lanterns for people to put up on the church steeple to let people know the British are coming on Paul Revere's midnight ride.

i think the answer is c, rivers.

explanation:

sometimes, borders fall along natural boundaries like rivers or mountain ranges. for example, the boundary between france and spain follows the crest of the pyrenees mountains. for part of its length, the boundary between the united states and mexico follows a river called the rio grande. the borders of four countries divide africa’s lake chad: niger, chad, cameroon, and nigeria.

the use of email to replace slower surface mail.