Physiologists have found that human brain hair that controls advanced cognitive functions such as memory and thinking has 6 layers under normal circumstances. If the fetus lacks an enzyme called "Cdk5", the development of the cerebral cortex will become extremely slow. The layer structure of the normal cerebral cortex cannot be eventually formed, which seriously affects children's cognitive development. Some medical professionals speculate that using this discovery will hopefully develop drugs and generally improve students' learning ability. If the following is true, which one cannot question the above speculation? ['The human body cannot absorb the externally-added "Cdk5" enzyme.', "Students' learning ability is mainly acquired through acquired learning.", 'After the critical period of adolescent brain development, supplementation of "Ddk5" enzyme does not work (partially acknowledged, cannot be questioned).', 'Some people with high pulse "Cdk5" enzyme content do not have outstanding learning ability.']