Question

old pennies become dull when the copper on the outside combines with oxygen in the air. this can take many years. the result is a new chemical called copper oxide. when you put pennies dropdown with options: into, around, beside a vinegar and salt solution, the acetic acid dissolve the copper oxic rk together. they s part of the solution.

Explanation:

The sentence structure here requires a preposition that indicates placing the pennies inside the vinegar and salt solution. "into" is the correct preposition as it shows movement towards the interior of the solution, while "around" and "beside" do not fit the context of putting the pennies in the solution to dissolve the copper oxide.

Answer:

into