Minimize the objective function 900y_(1)+700y_(2)+800y_(3)+180x_(11)+170x_(21)+160x_(31)+210x_(12)+230x_(22)+220x_(32)+140x_(13)+120x_(23)+130x_(33). Given the constraint type A:x_(11)+x_(21)+x_(31)<=3y_(12), what is the appropriate next step to solve this problem?

A) Simplify the objective function.
B) Substitute the constraint into the objective function.
C) Solve the constraint for y_(12).
D) Substitute the constraint into the objective function and solve for the optimal values of the variables.