Method 1: directly add-1,-2,0 to generation B, and you can judge. Note that the answer is-1. Let x=0, then f (x) =-1/2x 2. At this time, the function is increasing in this interval, so 0 can't be in the answer interval, so A.B can be excluded. Similarly, substitute-1 or -2 to get the answer.

Method 2: direct problem solving method. Given f'(x)=-x+b/(x+2), if f(x) is a decreasing function in this interval, then the derivative of the function must always be negative in this interval, that is, f' (x) =-x+b/(x+2).