Why Did Cattell and Horn Propose That Spearman's G Factor

Why did Cattell and Horn propose that Spearman's g factor, and Thurstone's concept of primary mental abilities should be subdivided into fluid and crystallized intelligence?

A) to distinguish between the ability to solve abstract relational problems and problems that depend on school knowledge and life experiences
B) to distinguish between the ability to solve abstract relational problems and information-processing skills
C) to distinguish between the ability to solve problems requiring flexible and rigid states of thought
D) to distinguish between the ability to solve abstract relational problems and problems requiring "street smarts"