A. <span>Exposure to an antigen
Exposure to viruses and antigens led to the foundation of modern vaccination. Vaccination introduces an antigenic material that helps the ability of the immune system to adapt a specific pathogen, therefore leading to a long-term immunity.</span>
The answer for this is "All of above"
Apomictic refers to plants and it means the offspring is genetically identical to the parent. Automictic means that gametes fuse after meiosis - which means offspring are genetically not identical to parents.
Astronomy is the scientific study of celestial objects. (Such as stars, planets, comets, and galaxies)