Consider an experiment to investigate the effectiveness of different insecticides in controlling pests and their impact on the productivity of tomato plants. What is the best reason for randomly assigning treatment levels (spraying or not spraying) to the experimental units (farms)? (a) Random assignment allows researchers to generalize conclusions about the effectiveness of the insecticides to all farms. (b) Random assignment will tend to average out all other uncontrolled factors such as soil fertility so that they are not confounded with the treatment effects. (c) Random assignment eliminates the effects of other variables, like soil fertility. (d) Random assignment eliminates chance variation in the responses. (e) Random assignment helps avoid bias due to the placebo effect.