Answer:
A, B
Explanation:
The Y-axis is plotting only the downstream values this particular river is an outlier on the y-axis. What is an outlier? An outlier is a entry that varies greatly from the rest of the entries. This variation could be lower or higher than the average so one answer is B. A residual is the offset of a point in respect to its regression line (line of best fit) having a high residual means that it's a outlier so A is also correct.
Hope this helps!
Ben