Please help me with exam practice question, brainliest for best answer!!
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In the upper reaches (also known as the source) the gradient is at its steepest because of vertical erosion. This changes further downstream as lateral erosion (side to side) from the river and deposition start to replace vertical erosion as the dominant process. Finally, the river really flattens out as it approaches the mouth as deposition become dominant.

b is the right answer for this question.