The processes of photosynthesis and cellular respiration are reciprocal processes because:

Photosynthesis makes the glucose that is used in cellular respiration to make ATP. The glucose is then turned back into carbon dioxide, which is used in photosynthesis. While water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water.

A dichotomous key is a way to figure out what something is by picking between two possible descriptions and choosing whichever one is correct. From there, you're told to go to another description (usually labeled with numbers) and you eventually get to the answer.