by the mass of the magnet lifting the metal paper clips by the size of the magnet lifting the metal paper clips by the number of metal paper clips lifted by the magnet by the time the metal paper clips stay on the magnet
I gave my students this multiple choice question: A student sets up an investigation to test the strength of magnets. He has several magnets of different sizes, shapes, and masses. He uses the magnets to lift metal paper clips. How is the strength of a magnet defined in the investigation?

Only one answer is correct among these 4 choices:
- by the mass of the magnet lifting the metal paper clips
- by the size of the magnet lifting the metal paper clips
- by the number of metal paper clips lifted by the magnet
- by the time the metal paper clips stay on the magnet

Could you tell me which one is correct?

by the number of metal paper clips lifted by the magnet