This sentence is missing a ran as fast as they could go, out the door, down the street, and back home

NOUN

The sentence fails to express a subject noun, and jumps straight into the verb ran.  

A noun that works as a subject.

Explanation:

Actions need someone or something that performs them, that is to say, a noun, a doer. Without a noun that performs the main action of "ran", this sentence leaves room for doubt and confusion. So, in order to avoid ambiguity, this sentence needs a noun that works as a subject that performs the main action. In this case, to fill the blank with the pronoun "They" or any other noun in third-person plural would be right.

one is a red flower, and one is the sky when the sun sets.