identify the missing parts of the carbon cycle photosynthesis burning oil and coal respiration decomposers​

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

What best explains the inability for life to exist in earth early atmosphere

Two students are working together on an experiment that measures the effect of different liquid fertilizers on the thickness of plants’ stems. which is most likely to result in the greatest amount of error in their scientific experiment? a.one student measures the plant stem widths by using calipers. the other student measures the widths of the same plant stems again by using a different pair of calipers. b.one student measures the plant stem widths by using calipers. the other student types the measurements as they are taken into a computer spreadsheet. c.one student weighs liquids for the experiment by using a calibrated scale. the other student weighs liquids for the experiment by using a different calibrated scale.