If only we ____ the disease was curable then.[      ]

A. knew    

B. could know    

C. had known    

D. would have known

我国海盐的历史可以追溯到（）时的诸侯宿沙氏首创用海水煮制海盐。

A.炎帝

B.夏

C.商

D.西周