Helping verbs are verbs that help the main verb in a sentence by extending its meaning. They can also add detail to how time is conveyed in a sentence. As a result, helping verbs are used to create the most complicated verb tenses in English: the progressive and the perfect aspects.
Don't quote me on this, but it might be C
C) Biologists backed up these ideas with sketches and models based on known animals.
A prepositional phrase contains a preposition and usually a noun. Your answer would be the first choice.
example:
I went over the hill. "Over the hill" is a prepositional phrase.
