(1) scientists generally agree that earth is getting warmer due to what's called the greenhouse effect. (2) a greenhouse stays warm because sunlight and heat coming through the glass roof are mostly trapped. (3) the heat doesn't radiate back into the atmosphere because it's held in by the glass. (4) something like that happens on earth. (5) the atmosphere acts like a blanket that keeps the sun's heat from leaking away into space. which sentence is the paragraph's topic sentence?

active voice: my father cut the tree.

passive voice: the tree was cut by my father.

active voice: people buy newspapers every day.

passive voice: newspapers are bought (by people) every day.

active voice: people have been using movies as political propaganda since world war ii.

passive voice: movies have been used as political propaganda since world war ii.

active voice: donna gave me my birthday present.

passive voice: i was given my birthday present (by donna). / my birthday present was given to me (by donna).

active voice: mom will leave the keys on the counter.

passive voice: the keys will be left (by mom) on the counter.

the words in parentheses are the agent of the passive voice and can be omitted when obvious, generic, or simply unnecessary.