Question

which part of the river has the most oxygen?
3rd-5th order streams
1st and 2nd order streams
higher order streams
none of the above

Explanation:

1st and 2nd order streams are smaller, faster-flowing headwater streams. Their high flow velocity and turbulent mixing with air allow for greater oxygen dissolution into the water compared to slower, larger (higher-order) streams which have more organic matter consuming oxygen and less turbulent mixing.

Answer:

B. 1st and 2nd order streams