What is generally true about essential elements and living organisms?

• A. All organisms require 25 of the 92 naturally occurring elements to survive.
• B. The elements carbon, hydrogen, nitrogen, and oxygen make up 20–25% of living mass.
• C. Given their low concentrations in nature, the toxicity of some elements is generally not a factor in the evolution of biological communities.
• D. Although all forms of life require iron, other elements are required only by certain species.