Let f be a differentiable function defined by z= f(x,y). If the directional derivative of the function at the point (1,2) in the direction of vector 1,0 is 2 and in the direction of vector 0,2 is 3, then the directional derivative of f at the point (1,2) in the direction of the vector 1,2 is: *Remember that the equality D_uf(x,y)= f(x,y)u is valid when u is a unit vector. Select one: a. 45 / 5 b. 45 / 5 c. -1 d. 1