19 DETAILS MY NOTES ASK YOUR TEACHER Bicycles. A local company that manufactures and sells bikes has found that its monthly revenue (in dollars) is given by R(x,y) = -5 x - 2.5 y - 5 xy + 500 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 (25,90) O (0,0) O (50,70) O (36,28) O (28,36) There is no critical point O (70,50) O (90,25) (b) Let D(x,y)=Rx(x,y)R,(x,y)-R."(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) > 0 O D(a,b) > 0 and Rxx(a,b) 0 O D(a,b) = 0 O D(a,b)