Which among the following is the correct conclusion for the gravitational field at distance r for two concentric shells of masses m1 and m² with radii r1 and r2?
1) The gravitational field is zero at distance r.
2) The gravitational field is non-zero at distance r.
3) The gravitational field is directly proportional to the masses m1 and m².
4) The gravitational field is inversely proportional to the radii r1 and r2.