│BD│=b, then the first two known conditions can be changed to A+B = 4 and A * B = 4, and a=b=2. Note that AC, BD and (AB+CD) can form a right triangle with AC as the hypotenuse, so there is

AB+DC=AC*cos∠CAB, so (AB+DC) × AC = (│ AB │+│ DC │) * │ AC │ cos ∠ cab.

=(│AB │+│DC │) & amp； Sup2 = a & sup2 = 4, choose C.