Detailed Instructions: You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Q: 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?
(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
A:
B