We have not discussed methods by which systems of first-order differential equations can be solved. Nevertheless, systems such as: (dx) /(dt) = -λ₁x (dy) /(dt) = λ₁x - λ₂y (dz) /(dt) = _______ What is the missing term in the system? a) λ₁z b) -λ₁z c) λ₂z d) -λ₂z