A plant nursery watered one season's seedlings by a slow drip instead of by hand, and the share of seedlings surviving to sale rose from three in five to four in five. The owner argues that the change in watering is responsible. Which of the following, if true, most strengthens that argument?

Better seed and a kinder season are the two explanations that would account for the improvement without the watering. One lot of seed for both seasons disposes of the first, and matching temperature and humidity records dispose of the second, so the drip is left holding the field. Cost, the mix of customers and early sales do nothing to eliminate either rival.