There are three charged balls: a) Can any two balls of these three repel each other?  

There are three charged balls:
a) Can any two balls of these three repel each other?  
b) Can any two balls of these three be attracted to each other?

a) If all balls have a charge of the same sign, then they can.
b) They cannot, because among three charged balls, two will have charges of the same sign.

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