a positively-charged wool sock and a neutral plastic bag a neutral cotton sweater and a negatively-charged plastic pellet a negatively-charged balloon and a positively-charged piece of paper a negatively-charged plastic strip and a negatively-charged piece of cloth
I gave my students this multiple choice question: Which pair of items will repel each other?

Only one answer is correct among these 4 choices:
- a positively-charged wool sock and a neutral plastic bag
- a neutral cotton sweater and a negatively-charged plastic pellet
- a negatively-charged balloon and a positively-charged piece of paper
- a negatively-charged plastic strip and a negatively-charged piece of cloth

Could you tell me which one is correct?

a negatively-charged plastic strip and a negatively-charged piece of cloth