I think suffix addition is what you're looking for
The correct option is C.
An effective summary must have some features which include the following:
It must serves as a mean of getting a quick information about the content of the text. It must provide accurate and precise presentation of the original text most important points. It must be paraphrased to show the author's original thoughts.<span />
I read this poem not long ago, and I think that excerpt means that the caged bird cant go anywhere, all his freedom is gone except for one thing, and that is singing. He sings because that is the only thing he can do that he is tied down to the cage.
I would have to say it acts as the subject of this sentence