Multi-choice problem: Choose from the options at the end, and answer how do you do this: To reinforce a paper project,

Pick your answer from:
-- cut a piece of cardboard in equal size to the paper you want to strengthen, and glue the cardboard to the back of the paper design.;
-- cut a piece of cardboard in equal size to the paper you want to strengthen, and glue the cardboard to the front of the paper design.;
cut a piece of cardboard in equal size to the paper you want to strengthen, and glue the cardboard to the back of the paper design.