Ms. Barton has four children. You are told correctly that she has at least two girls but you are not told which two of

Ms. Barton has four children. You are told correctly that she has at least two girls but you are not told which two of her four children are those girls. What is the probability that she also has two boys? (Assume that the probability of having a boy is the same as the probability of having a girl.)

(A) 1/4
(B) 3/8
(C) 5/11
(D) 1/2
(E) 6/11

[spoiler]OA=E[/spoiler]

Source: Manhattan GMAT