Here is the item again:

If his friends walked into the bookstore (A) Boris had a stack of car magazines as camouflage, (B) but he was really reading Better Homes and Gardens, studying decorating ideas (C) to spruce up his first apartment.

You correctly realized that a comma must follow bookstore. If his friends walked into ... is a subordinate clause. When a subordinate clause introduces a main clause, you connect the two with a comma.

To fix the problem, you chose to do this:

If his friends walked into the bookstore, Boris had a stack of car magazines as camouflage, but he was really reading Better Homes and Gardens, studying decorating ideas to spruce up his first apartment.

You have correctly employed Punctuation Rule 2:

Subordinate Clause + , + Main Clause.

Excellent choice!

Go to the next sentence.

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