Rivers flow downhill because of gravity are large bodies of contained water usually end in mountainous areas are usually made of saltwater

Rivers flow from high areas to low areas, until it empties into a larger body of water, like a lake, a larger river, or the ocean.

Hence, the correct answer is A. flow downhill because of gravity.

ń, wǒ shuō yīngyǔ, dàn wǒ huì shuō qítā yǔyán, rú xībānyá yǔ fǎyǔ yìdàlì yǔ hé pǔtōnghuà, suǒyǐ bùyào pànduàn, dànshì