---How long do you think it will be China becomes a developed country?---At least 20 years

问题补充：

---How long do you think it will be China becomes a developed country?

---At least 20 years, I guess.

A. that

B. until

C. when

D. before

答案：

A. that

B. until

C. when

D. before

解析：it+段时间+before… 句型意为：要过多久才……。所在句意为：你认为中国要过多久才能成为一个发达国家。It …until…意为“直到……才……”与句意不合。