Answer:
The four words are:
"During the Chemistry experiment..."
Step-by-step explanation:
The above words are the four words that make up the prepositional phrase.
A prepositional phrase is known to be a group of words which has a preposition with its object and other words that modify the object. Prepositional phrases can actually modify a noun or a verb.
Examples of common prepositions that actually begin prepositional phrases in sentences are: to, about, during, behind, after, for, of, at, before, under, with, over, with, and by.
The above sentences begin with a preposition and it is answering the question of when the smoke filled the class.