What is the simple subject of the following sentence "a virus is a microscopic organism."

How does the author use the character of the nurse to develop the social issue of gender inequality? the nurse’s willingness to take care of nora when nora was little demonstrates a woman’s ability to do difficult work. the nurse’s need to give up a child in order to have a job demonstrates a woman’s inability to support herself. the fact that the nurse’s child wrote to her as a young girl and as an adult demonstrates a woman’s ability to keep friends forever. the nurse’s assurance to nora that her children ask for her often demonstrates women’s role in health care.