According to Diego, what is the main reason or obstacle to influence Chinese students not to speak English well?
选项：

A:They are quite afraid of making mistakes.
B:They become very nervous and sometimes embarrassed to speak when they’re suddenly expected to start to speak.
C:They don’t want to try, because they are afraid of taking a risk.
D:They have been learning a lot of formal English, very focused on grammar and very focused on the written word.