A friend of mine asked me this question, that is asked in an entrance examination. It shouldn't be that difficult, but I fail to find a rigorous answer for it.
The figure shows three charges, that are fixed on a line, so that they can't move.
Now the question is, if $q$ is an amount of positive charge, then what are the charge of $A$, $B$ and $C$?
The possible answers are:
It would be nice if anyone could give a rigorous answer, because it seems at the moment that this question has to be solved by intuition, which is strange for an entrance examination.