Here's a problem to solve: Estuaries are dynamic ecosystems where salt water and fresh water mix. Usually this happens as fresh water flows from a river to an ocean. When would you expect the concentration of salt in the estuary water to be LOWEST near the mouth of the river?

Among the 4 following options, which is the correct answer?

- A: when the river flow is low and the tide is low
 
- B: when the river flow is high and the tide is low
 
- C: when the river flow is low and the tide is high
 
- D: when the river flow is high and the tide is high
 