Charles Spearman Inferred the Existence of a "G" (Or General)

Charles Spearman inferred the existence of a "g" (or general) factor in intelligence from the observation that

A) people's scores on intelligence tests correlate highly with measures of the size of their cerebral cortex.
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) people who do well on a test of one kind of intellectual ability generally do well on tests of other kinds also.
D) the mean score on intelligence tests is the same in all countries.