Carlisle is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. he purchases 2 different brands of planting soil from a local store. carlisle applies brand a to an area of the yard that receives full sunlight and brand b to another area of the yard that is in partial shade. he waters both areas daily. at the end of the study, carlisle concludes brand a is more effective in growing blueberry bushes. why is his conclusion not valid?