Final answer:
The prepositional phrase in the sentence is 'about Cheng I sao'.
Step-by-step explanation:
The prepositional phrase in the sentence is about Cheng I sao. A prepositional phrase is a group of words that begins with a preposition and ends with a noun or pronoun. In this case, the preposition is about and the noun phrase is Cheng I sao. The prepositional phrase functions as an adverbial phrase, providing additional information about what is known.