What would have happened,_____ as far as the ri- verbank?

[      ]

A. Bob had walked

B. if Bob should walk

C. had Bob walked

D. if Bob walked

亚油酸是( )。

A．18碳2烯酸
B．22碳4烯酸
C．24碳6烯酸
D．18碳4烯酸