If the cost of debt is the lowest choice among financing options, would increasing our percentage of debt always reduce our cost of capital?

If the cost of debt is the lowest choice among financing options, would increasing our percentage of debt always reduce our cost of capital? Why or why not?
What is capital structure and how important it is? What is the basic goal of financial management with regard to capital structure? How is the cost of capital related to capital structure?
answer these two questions 200 words each