Rigid frames, the unit load method. The frame in Figure $1$ is composed of three beams with constant bending stiffness El$.$ The frame is loaded with a vertical point load F at D$.$

$1.$ Determine the vertical displacement of D$.$

$2.$ Determine the horizontal displacement of D$.$

$3.$ It is desired to minimize the vertical displacement of point D$.$ Where would it be most appropriate to double the bending stiffness El$,$ on section AC or CD$?$ Justify.