For one kindergarten class in his district, a researcher determines which children already can read simple words and which children cannot upon entering kindergarten. The children are followed until third grade, at which point they are tested to determine the grade level at which they are reading. Those children who were reading simple words upon entering kindergarten are found to be reading at a higher level than those who could not read simple words upon entering kindergarten. Fill in the Blank: The researcher . Select one: a. can conclude that children should be taught to read in preschool, as there are clear benefits to reading early. b. needs to check the reading level of the children's parents. c. cannot establish a cause-and-effect relationship because the study did not use a random sample of kindergarten students. d. finds these results beneficial, as there may be confounding variables in this study. e. needs to retest in sixth grade or no conclusions can be reached.