A state standardized test is administered to all seventh graders within the first week of classes. One of the test administrators decides to have the students take the test again a month later to run some analysis on the test itself. The results show significant differences between the first administration and the second; some students have improved and others have declined. There does not seem to be an obvious reason for the differences in scores. This instrument may be vulnerable to a. acquiescent response set. b. social desirability bias. c. random error. d. systematic error