Use the branch-and-bound method to solve the binary linear programming model:
max, 8x₁ + 11x₂ + 6x₃ + 4x₄ s.t.
6.7x₁ + 10x₂ + 5.5x₃ + 3.4x₄ ≤ 19 , 8x₁ + 2x₂ ≤ 19 , x₁.
a) 10
b) 8
c) 6
d) 12