— when has the country been open to international trade?— 1978 I suppose. A. SinceB. In

问题补充：

— when has the country been open to international trade?

— 1978, I suppose.

A. Since

B. In

C. From

D. After

答案：

(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?