Read the excerpt from "Mother Tongue."

"Yet some of my friends tell me they understand 50 percent of what my mother says. Some say they understand 80 to 90 percent. Some say they understand none of it, as if she were speaking pure Chinese. But to me, my mother's English is perfectly clear, perfectly natural. It's my mother tongue. Her language, as I hear it, is vivid, direct, full of observation and imagery. That was the language that helped shape the way I saw things, expressed things, made sense of the world."

How does Tan build a central idea of her story in the excerpt?

A. Tan describes her friends' understanding of her mother to support the idea that nonstandard forms of English should only be used in the home.
B. Tan uses numerical data to support the idea that nonstandard forms of English are on the decline and being replaced by one standard form.
C. Tan tells a story about her mother's life to support the idea that learning the language of a new country comes naturally to most people.
D. Tan discusses her mother's English to support the idea that the language of one's childhood is a person's deepest, truest form of expression.