What Environmental Factor Makes It Very Difficult for Organisms That

What environmental factor makes it very difficult for organisms that live in estuaries?

A) the movement of water as the river flows into the ocean
B) the constant fluctuation in the salinity of the water as the salt water and fresh water mix
C) the fluctuating nutrient levels as the river brings different levels of nutrients to the estuary
D) the change in water levels as the tide ebbs and flows
E) the change in the temperature of the water since the river water is colder than the ocean