Which part of cellular respiration must occur before any of the other steps can occur
a. pyruvate processing
b. krebs cycle
c. glycolysis
d. electron transport chain

Glycolysis must occur before any of the other steps.  

Explanation:  

Glycolysis is the initial step in breaking down the 6-carbon sugar like glucose into 3-carbon organic molecules and finally results into the production of the two molecules of the 3-carbon compounds which is called "pyruvate". Pyruvate is converted into acetyl coenzyme A during pyruvate processing and enters into citric acid cycle or krebs cycle and the reduced compounds like NADH and FADH2 will be oxidized to transport electrons and protons in order to generate the ATP molecules.

volcanoes (an event in the geosphere) release a large amount of particulate matter into the atmosphere. these particles serve as nuclei for the formation of water droplets (hydrosphere). rainfall (hydrosphere) often increases following an eruption, stimulating plant growth (biosphere).