Xavier is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. he purchases two different brands of planting soil from a local store. xavier 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 sunlight. he waters the area with brand b daily; he waters the area with brand a every other day. at the end of the study, xavier concludes brand a is more effective in growing blueberry bushes. why is his conclusion not valid?