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Explanation:

1. The materials are physically combined because all of the materials including the sand, salt, and iron fillings are their own substances and they maintain their original form. This is a mixture and they aren't chemically combined.

2. To separate each of the materials physical means would be used because since they're their own individual substances and have kept their original form they're mixed and not chemically combined.

3. The iron and oxygen atoms in rust are chemically combined because they are no longer individual substances and haven't kept their original form, so therefore they're a chemical combination.

a. the production of carbon dioxide through the process of photosynthesis

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.