A Researcher Carries Out a Study Examining the Effects of a New

A researcher carries out a study examining the effects of a new reading program on reading achievement as measured by a standardized test. He chooses two first-grade classes to carry out the study. After the study is done, he finds that group who received the new reading program performed better than the control group. He also discovers, however, that the group he chose to receive the new program had better prior reading performance. An extraneous variable in this study is:

A) Prior reading performance
B) The type of reading program
C) Reading achievement standardized test scores
D) Grade level