— I went to see you yesterday evening. But you weren't in. Where were you then?

— I            a walk by the lake with my husband. [ ]

A. was having

B. am having

C. have had

D. have

各国应该集中生产并出口那些充分利用本国充实要素的产品，进口那些需要密集使用本国稀缺要素的产品。这一结论依据的是( )理论。

A．亚当·斯密的绝对优势
B．大卫·李嘉图的比较优势
C．赫克歇尔和伯蒂尔·俄林的生产要素禀赋理论
D．克鲁格曼的规模经济