What Was Spearman's Evidence in Favor of a G Factor

Multiple Choice

What was Spearman's evidence in favor of a g factor in intelligence?​

A) Certain genes are more common among people who score high on IQ tests.
B) ​Mean scores on IQ tests have been rising from one generation to the next.
C) ​Most people who do well on one mental test do well on other ones also.
D) ​Scores on IQ tests correlate positively with measurements of brain volume.