In Mattie's mind, Blanchard's balloon—“a yellow silk bubble escaping the earth”—symbolizes freedom, particularly an escape from her home, where she feels as if she's trapped and treated like a child.
The theme of Sonnet 43 by Elizabeth Barrett Browning is:
C. The speaker loves her beloved in every possible way.
Browning’s Sonnet 43, which is entitled “How Do I Love Thee?” speaks about the endless ways of how she loves her beloved.
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