In which of the following pairs, both species have not nearly the same size ?

Explanation

Thymus is small at birth, it increases in size during childhood and reaches maximum size at puberty. During adulthood, it shrinks to its size at birth.
In old person thymus gland is degenerated, resulting in a decreased production of thymosin: Thymosin plays an important role in immune developments. Thus, immune response against infections of old people becomes weak.