All three vertices of an equilateral triangle are on the parabola y=x^{2}, and one of its sides has a slope of 2 . The x-coordinates of the three vertices have a sum of m / n, where m and n are relatively prime positive integers. What is the value of m+n ?
Answer Choices
A. 14
B. 15
C. 16
D. 17
E. 18