If altitude bubble is provided both on index frame as well as on telescope of a theodolite, then the instrument is levelled with reference to_________________?

1. altitude bubble on index frame
2. altitude bubble on index frame if it is to be used as a level
3. altitude bubble on telescope
4. altitude bubble on telescope if it is to be used as a level
The correct answer is?
A. only (1)
B. both (1) and (4)
C. only (3)
D. both (2) and (3)