Complete the annoyed customer's complaint about a delivery. Select the grammatically correct phrase.

"When I lifted the heavy package, the _____ completely fell out!"

The correct answer is bottom of the box.

For inanimate objects (things that aren't alive, like a box), we generally use the of structure to show possession or parts, rather than an apostrophe and "s". We say "the bottom of the box" instead of "the box's bottom."