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>
Answer:
D
Explanation:
It is often like this: cell->tissue->organ->system
Cell wall is the outer permeable covering in all pants and almost all prokaryotes ! cell wall are mainly made up of cellulose , sometime by chitin!!
The last one, the planets formed at the same distance from the sun.
