Bicycles. A local company that manufactures and sells bikes has found that its monthly revenue (in dollars) is given by R(x,y) = -4 x2 - 2.5 y2 - 4 xy + 400 x+ 320 y where x is the number of mountain bikes and y is the number of city bikes that it sells each month. (a) The critical point of R(x,y) is O (30,40) O (50,60) O There is no critical point O (40,30) O (0,0) O (90, 25) O (60,50) O (25,90) (b) Let D(x,y)=Rxx(X,y)Ryy(X,y)-Rxy"(x,y). Suppose that a is the number of mountain bikes and b is the number of city bikes that should be sold each month to ensure a maximum monthly revenue. Which of the following statements is true? O D(a,b) 0 and Rxx(a,b) > o O D(a,b) > 0 and Rxx(a,b) o