Find all critical numbers of the function f(x)=x-72x. Then use the second-derivative test on each critical number to determine whether it leads to a local maximum or minimum. Select the correct choice below and, if necessary, fill in the answer box(es) to complete your choice. OA. The critical number(s) is/are at x= There is no local maximum and no local minimum. (Type an integer or a simplified fraction. Use a comma to separate answers as needed.) B. The critical number(s) is/are at x=. There is a local maximum at x= There is no local minimum. (Type an integer or a simplified fraction. Use a comma to separate answers as needed.) OC. The critical number(s) is/are at x= There is no local maximum. There is a local minimum at x= (Type an integer or a simplified fraction. Use a comma to separate answers as needed.) OD. The critical number(s) is/are at x= There is a local maximum at x= There is a local minimum at x= (Type an integer or a simplified fraction. Use a comma to separate answers as needed.) OE. There are no critical numbers.