The equation of the parabola whose focus is the point ( 0 , 0 ) and the tangent at the vertex is x − y + 1 = 0 is
a. x² + y² − 2xy − 4x − 4y − 4 = 0
b. x² + y² − 2xy + 4x − 4y − 4 = 0
c. x² + y² + 2xy − 4x − 4y + 4 = 0
d. x² + y² + 2xy − 4x + 4y − 4 = 0