Suppose that a state offered voluntary standardized tests to all its third graders and that these data were used in a study of class size on student performance. which of the following would generate selection bias?

a)schools are unable to control for unobservable student ability

b) scores on standardized test are usually recorded with error

c) class size is simultaneously determined with student performance

d)schools with higher-achieving students could be more likely to volunteer to take the test