Final answer:
Among the options provided, Nitrogen (D) and Calcium (B) are the two substances that are among the most abundant elements in living things. Nitrogen is one of the top four elements essential for life, while Calcium is important but less abundant.
Step-by-step explanation:
The question asks which two substances are among the six most abundant elements in living things. The four most common elements in all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). These elements are the fundamental components of living matter, forming nucleic acids, proteins, carbohydrates, and lipids that are vital for life.
Given the options of Helium (A), Calcium (B), Chlorine (C), and Nitrogen (D), we can see that Nitrogen is one of the most abundant elements in living things. The other options provided are not part of the six most abundant elements in living organisms. Therefore, the two substances from the options provided that are among the six most abundant in living things are:
While Nitrogen is one of the four basic building blocks of life, Calcium, though less abundant than the top four, still plays a critical role in cellular physiology as a secondary messenger and structural element in bones.