Two identical charges are placed at the two corners of an equilateral triangle . The potential energy of the system is $\;\sqcup\;.$ The work done in bringing an identical charge from infinity to the third vertex is

Question

$(a)\;\sqcup\qquad(b)\;2\sqcup\qquad(c)\;3 \sqcup\qquad(d)\;4 \sqcup$