So difficult __________ it to work out the problem that I decided to ask Tom for advice.

A．I did find

B．did I find

C．I have found

D．have I found

大豆中含有对人体健康有益的物质有（）更适于老年人食用

A.大豆油

B.磷脂

C.植物固醇

D.大豆异黄酮