After looking at many new cars, I found ________ which I would accept just as suitable.

--Can I help you?  --I'd like to buy a present for my father's birthday, ___________ at a proper price but of great use.

 —Have you finished your work yet?—No, I’ll finish it in ________ fifteen minutes. 

Our classroom is very clean. Do you know who＿＿＿cleaned it? 

—Do you think playing cards is beneficial to our study?—No，if ________，it can help us kill time when we have nothing to do.