Flagellum. This is the only thing I could come up with because you did not provide an image.
Viruses need a host because a host is just like the owner of the virus "business". think of it as a company. the company has a boss- aka the person who made the company (this is the host) and the boss also tells the employees (bacteria etc) what to do and how to do it. without the host, the virus will die. :)
Explanation:
plants that are shorter and need little to no soil are most efficient. Lichens, which are part fungus and usually part algae, don't need extensive root or water-transportation system
I would say it is B!... there are dense, positive charged mass in the center of the atom
