1. Communities (Biotic factors) and Abiotic factors of an environment form an a. Community
b. Population
c. Ecosystem
d. Species

C.) Ecosystem

Explanation:

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Calcium and argon have the same atomic mass but have a different atomic mass because the atomic number depends upon a number of protons which is mainly different. Argon has 18 number of protons which is its atomic number and calcium has 20 number of protons. A number of neutrons and protons are different. But atomic mass (A=Z+n)  is the same. Atomic mass is the sum of the atomic number number of protons and number of neutrons.