Answers are: b) It help cells make protein ; c) It provides a form of usable nitrogen.
Nitrogen cycle is important for all living things as nitrogen is an important constituents of amino acids which makes protein, nucleic acids such as DNA and RNA, ATP and other compounds which are essential for life. During Nitrogen cycle, nitrogen is fixed into ammonia by bacteria. Ammonia is converted into nitrite and then to nitrate. Nitrate is the most usable form of nitrogen by plants. Animals feed on plants to get nitrogen. After death of plants and animals, they decomposed by bacteria and nitrogen is cycled back to atmosphere.