Read the excerpt from The Building of Manhattan. It was a monumental task: to design the world's tallest building, create as much usable rental space as possible, plan so it could be built within 18 months, and still have a handsome building. In the end, architect William Lamb worked on fifteen designs before his plan "K" was finally chosen.

Which words cannot be changed when paraphrasing this text?

“monumental task”
“handsome building”
“fifteen designs”
“usable rental space”