Sarah is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. she purchases two different brands of planting soil from a local store. sarah applies brand a to 1 area of the yard and brand b to another area of the yard, both of which receive full sunlight. she waters the area with brand b daily; she waters the area with brand a every other day. at the end of the study, sarah concludes brand b is more effective in growing blueberry bushes. why is her conclusion not valid?