Choose the sentece that is written in 3rd person. question 21 options: i am going to tell you about the major causes of homelessness in america. global warming can be decreased if you try to carpool, use less energy, and make sure you use quality gasoline. exercise promotes a healthy lifestyle by reducing the effects of stress, managing weight, and strengthening the immune system. if we work together, we can prevent global warming by carpooling, recycling, and using environment friendly cleaner.

Exercise promotes a healthy lifestyle by reducing the effects of stress, managing weight, and strengthening the immune system.

Third person is when the speaker is outside of the story and not giving directions to someone. In this sentence, the speaker is telling explaining how exercise promotes a healthy lifestyle. The speaker does not use and first or second person pronouns.

Looking at the pronouns can help you identify first, second, and third person. 
First-person pronouns are: I, me, my, myself.
Second-person pronouns are: You, your, yourself (Most often second person is used for instructional manuals. Think cookbooks and car repair guides.)
Third-person pronouns: He, She, It, They 

Keep in mind that We is a pronoun that combines I and You. Thus eliminating the last option.