Identify a true statement about test-retest reliability estimates.
a) This measure is appropriate when evaluating the reliability of a test that purports to measure something that is relatively stable over time.
b) This estimate allows a test user to estimate internal consistency reliability from a correlation of two halves of a test.
c) This measure is used when it is impractical or undesirable to assess reliability with two tests or to administer a test twice.
d) This estimate is obtained by correlating two pairs of scores obtained from equivalent halves of a single test administered once.