If b < 1, then a*b would be smaller than a because b is a decimal; a+b, on the other hand, would be bigger than a because b is positive. This would make the answer A. However, if b > 1, then a*b would be larger than a+b. For instance, if b=2, then a=6, and 2+6 would be less than 2*6. This would make the answer B. Since both cases exist, the answer is D.