---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

---How long do you think it will be _____China becomes a developed country?
---At least 20 years, I guess

1. A.
   that
2. B.
   until
3. C.
   when
4. D.
   before