A cell membrane is c.selectively permeable because it regulates what materials enter the cell.
C. were conditioned to salivate in response to the sound of a bell.
We produce carbon dioxide, plants take it in for photosynthesis.
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The beginning products (plants take these in to start photosynthesis) are h2o and co2
(Water and carbon dioxide )
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So you could say carbon dioxide is a beginning product in photosynthesis.
Hope this helps!