Service tax bill booking

Student (b) (875 Points)

17 December 2011  

 Under Service tax laws, which of the following bills are to be recorded for cenvat credit?
1.Service rendered & bill has been raised by company.
2.Service rendered but bill has not been raised by co.
3. service availed and bill has been recd. by co.
4.Service availed but no bill has been recd.

what is the logic?

thanks