---How long has your brother been working in this factory?-he dropped from school in 20

问题补充：

---How long has your brother been working in this factory?

---he dropped from school in .

A. When

B. Until

C. Since

D. Before

答案：

A. When

B. Until

(答案→)C. Since

D. Before

解析：考查连词。A当……时候；B直到；C自从；D在……之前。句意：——你哥哥在这家工厂工作了多久？——自从他从学校辍学。故C正确。