Four point masses are placed at four corners of a square as shown. When positions of m and 2m are interchanged, which of the following is true ?

1. gravitational field strength at centre will increase 
2. gravitational field strength at centre will decrease 
3. gravitational potential at centre will remain unchanged 
4. gravitational potential at centre will decrease

(1) and (3)

On exchanging masses, potential at centre remains unchanged, but field changes.