Charles spearman inferred the existence of a "g" (or general) factor in intelligence from the observation that a. people who do well on a test of one kind of intellectual ability generally do well on tests of other kinds also. b. people who score high on a test one day are likely to score high again if they take the same test another time c. the mean score on intelligence tests is the same in all countries. d. people's scores on intelligence tests correlate highly with measures of the size of their cerebral cortex.