What is the origin of today's marathons?

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.

climate change is the big reason to the decline of the indus valley which also caused floods. so i dont know what answer would best fit the description thats just what i found online! have an awesome

where is the i can't with a multiple choice question without the choices