Challenge 2: Losing the Original Circles After Applying Union — Solution Walkthrough

The answer:

Per this chapter's own warning box, Union (like most standard
Pathfinder operations) is destructive — it replaces the original
overlapping shapes with a single new resulting path. Once applied, the
two original circles no longer exist as separate, individually
selectable objects; they've been merged into one continuous outline.

Why the colleague can no longer select just one circle:

There is no longer "one circle" to select — the Union operation
combined both circles' outlines into a single new path the moment it
was applied. What's selectable now is only the merged result, not
either of the two shapes that went into it.

How to avoid this in the future:

Per this chapter's own recommendation, duplicating the original shapes
before applying a Pathfinder operation preserves a separate, unmerged
copy that can still be edited individually later. Keeping that
duplicate hidden or set aside means the merged result can be discarded
and rebuilt from the originals if it turns out changes are still
needed.

WHY THIS WORKS AS AN ANSWER
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This exercise checks that Union's destructive nature is correctly
understood as the cause of the lost original shapes, and that
duplicating shapes beforehand is recognized as the actual preventive
measure.