Explanation:
A petrochemical is any chemical manufactured from crude oil and natural gas as distinct from fuels and other products (Speight, 2014, 2019a), derived from crude oil and natural gas, and used for a variety of commercial purposes. The definition has been broadened to include the whole range of organic chemicals.
The common noun here is "frog" as the only other noun, Many, is not a common noun - it's a proper noun.
"frog" here is the object of the sentence: it's the object of the verb "caught" of which the subject is Mary
