An estuary is
a.
a freshwater biome
b.
an area with a low concentration of minerals and nutrients
c.
characterized by deep water
d.
an ecotone

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

you didn't give any options but heres what i can tell you: an inference is likely to change based on additional observations. the latter is something that you have seen or sensed using your sensory organs while the former is like a conclusion out from what you have observed.