Karu is two months old and cannot yet sit upright on his own. Karu's inability to sit on his own is most likely due to which of the following?

• A. Karu suffers from an enlarged medulla.
• B. Karu's cerebellum has not yet fully developed.
• C. Karu has a lesion in his midbrain.
• D. Karu's reticular formation failed to form properly during prenatal development.