What do fires, hurricanes, and other natural disturbances result in?
a. commensalism
b. mutualism
c. primary succession
d. secondary succession

The answer to your question is D - secondary succession

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds

the missing space is phylum