What is the difference between an embryo and a foetus

Why do we perceive objects as retaining their brightness under different lighting levels?

In what cells is the human genome located?

What is genetic variation? -(a) each individual in a population has slightly different genes. -(b) some individuals in a population have more genes than others. -(c) each population of a species has slightly different genes. -(d)some populations of a species have more genes than others.