Christina conducts a study on the effects of a reading comprehension course to help first- graders who aren't reading at their grade level. She obtains a sample of 30 children from an inner city elementary school, and 30 children from a school in the suburbs. The 30 inner-city children are the control group and don't get the treatment, and the 30 children from the suburbs are the experimental group and get the treatment. Six months later, the children in the treatment group improved markedly in their reading, whereas the children in the control group did not. Christina's study suffers from which threat to internal validity?
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A.Instrumentation
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B.Selection
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C.Mortality
- D.Diffusion of treatment
For this assignment, I need to provide the correct answer as well as why the others are wrong. I have been unable to figure out the correct answer.