What Is the Main Reason That People Who Test Positive

What is the main reason that people who test positive for the genes associated with Alzheimer's disease do not always develop the disease?

A) Psychological and biological stressors interact with physiological processes to produce Alzheimer's disease.
B) The screening tests are often inaccurate.
C) Some forms of the disease are subclinical, in that they never manifest themselves in overt behaviour.
D) We still know so little about the brain processes involved.