Final answer:
The diversity in experiences and lifestyles and the decline in cognitive abilities make it difficult to study middle adulthood.
Step-by-step explanation:
The main reasons that make it difficult to study middle adulthood are the diversity in experiences and lifestyles (B) and the decline in cognitive abilities (D).
Middle adulthood is a stage of life where individuals have different experiences and lifestyles. This variation makes it challenging for researchers to generalize findings across the entire population.
Additionally, during middle adulthood, there is a decline in cognitive abilities, specifically in fluid intelligence, which involves information processing, reasoning, and memory. This decline can make it difficult to accurately assess cognitive functioning in this age group.