Complete the sentence. is thought to be most responsible for the salinity of the oceans.

a.) earthquakes stirring up the ocean floor
b.) chlorine gas combined with sodium from earth's rocks
c.) volcanoes releasing gas
d.) natural run-off from mountain ranges

The best and most correct answer among the choices provided by your question is the fourth choice or letter D. Natural run-off from mountain ranges is thought to be most responsible for the salinity of the oceans.

After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.