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 into a vinegar and salt solution, the acetic acid and salt work together. they dissolve the copper oxide. it becomes part of the solution. when you put steel paper clips in the solution, some of the iron in the steel dissolves as well. the copper in the solution and the iron that’s left in the paper strongly attracted to each other. so the cop cone up sticking to the paper clip. clip cup

Explanation:

Looking at the sentence "The copper in the solution and the iron that’s left in the paper \underline{\quad} are strongly attracted to each other", we need to choose the correct word from the options (clip, cone, cup). From the context, we know we put steel paper clips in the solution, so the iron is left in the paper clip.

The text is about an experiment with paper clips (steel) in a solution. The iron is part of the paper clip, so the correct word to fill in the blank is "clip" as it refers to the paper clip where the iron remains.

Answer:

clip