The Main Evidence for the Existence of a "G" Factor

The main evidence for the existence of a "g" factor in intelligence is the fact that

A) young adults ordinarily get higher test scores than do either children or older adults.
B) identical twins usually get similar scores on intelligence tests.
C) intelligence tests have several parts, and people who do well on one part generally do well on other parts also.
D) people who do well on an intelligence test today will probably do well when they take a similar test again later.