— when has the country been open to international trade?
— 1978, I suppose.
A. Since
B. In
C. From
D. After
网友回答
(答案→)A. Since
B. In
C. From
D. After
解析：
A。根据句中的现在完成时态可知，此题填since最佳。Since when的意思是“从什么时候开始”，又如：Since when have you lived here?你从什么时候起住在这里的?