Surveying - Engineering

Q1:

Pick up the correct statement from the following :

A the lines of sight while observing back sight and fore sight lie in the same horizontal plane

B the staff readings are measurements made vertically downwards from a horizontal plane

C the horizontal plane with reference to which staff readings are taken, coincides with the level surface through the telescope axis

D all the above.

ANS:D - all the above.

Will the measurements be always vertically downwards, as in operation B. Can anyone please tell me?