1. The problem asks: Why are viruses considered nonliving things? 2. To answer this, we need to understand the cell theory, which states that all living things are made of cells. 3. Viruses are made of DNA and proteins but do not have a cellular structure. 4. Since viruses lack cells, they do not meet the criteria of living organisms according to cell theory. 5. Therefore, the correct reason is: "Viruses are made of DNA and proteins, but not cells."