The diagram shows the arrangement of three small uniformally charged spheres, `A,B` and `C

The diagram shows the arrangement of three small uniformally charged spheres, `A,B` and `C`. The arrow indicate the direction of the electrostatic forces acting between the spheres (for example, the left arrow on sphere `A` indicates the electronstatic force on sphere `A` due to sphere `B`). At least two of the spheres are positively charged. Which sphere, if any, could be negatively charged?