To determine the root cause of contamination
there are 2 verbs : 1. writing , 2. learn
as verbs are noting but action words
Our English teacher helps us
Amy Tan (author of the Joy Luck Club) has written an absolutely terrific piece on what a well educated daughter (Tan) thinks of her mother's "spirited" English. It is an essay that is a masterpiece of its kind.
She explains in detail why her mother's English and how it is written doesn't matter. Her mother has other qualities that her language emphasizes. What matters is how well her mother is able to express herself ignoring all the usual rules of syntax.
From Tan's description, I have to say that C is the best answer.
If the lady speaks only Chinese, the meaning of the phrase means absolutely nothing. It is just sounds. D is wrong.
B is possible, but it would not be true for every idiomatic phrase. So I wouldn't pick B.
A has the same problem as D. I would stick with C