.
_____ the book where it was ______.
| A．Lay, laying | B．Lie, lying | C．Lay, lying | D．Lie, laying |
Only when the villagers were faced with water shortage___ how serious the problem of water pollution was．
| A．did they realize | B．will they realize | C．they did realize | D．were they realized |
Only by practicing every day ___________ English well.
| A．can we learn | B．we can learn | C．did we learn | D．learn we |
Only when 1 was fourteen__________the importance of study．
| A．I realized | B．did I realize | C．had I realized | D．I had realized |
Only after the plane had taken off _________ that he missed his parents so much.
| A．he found | B．he had found | C．did he find | D．was he found |
Only when I left my parents for Italy __ how much I loved them.
| A．I realized | B．I had realized |
| C．had I realized | D．did I realize |