Which of the following versions of sentence 2 (reproduced below) most effectively sets up the main claim of the passage? But when exactly in a person’s life does it become difficult to master the rules that govern a language?

A. (As it is now)

B. Which languages are said to be most difficult for adult native-English speakers to learn?

C. But why is the grammar of a foreign language so difficult for adult learners to master?

D. How did competency in grammar come to be such an important marker of language fluency?

E. But how has the rise of text messaging and tweeting contributed to an increase in obvious grammatical errors?