Halve the mass of m_{1} and halve the distance between m_{1} and m_{2}. Halve the mass of m_{1} and double the distance between m_{1} and m_{2}. Double the mass of m_{1} and halve the distance between m_{1} and m_{2}. Double the mass of m_{1} and double the distance between m_{1} and m_{2}.
I gave my students this multiple choice question: Which of the following actions will increase the gravitational force between two masses, m_{1} and m_{2}, by the greatest amount?

Only one answer is correct among these 4 choices:
- Halve the mass of m_{1} and halve the distance between m_{1} and m_{2}.
- Halve the mass of m_{1} and double the distance between m_{1} and m_{2}.
- Double the mass of m_{1} and halve the distance between m_{1} and m_{2}.
- Double the mass of m_{1} and double the distance between m_{1} and m_{2}.

Could you tell me which one is correct?

Double the mass of m_{1} and halve the distance between m_{1} and m_{2}.