The difference of level between a point below the plane of sight and one above, is the sum of two staff readings and an error would be produced equal to

ANS:B - twice the distance between the zero of graduation and the foot of the staff

The minor distance between the bottom graduation and staff bottom (graduation is usually like E shapes on staff, each arm of E equal to 1cm on staff) , For each station we need to take 2 reading at least FS & BS, and for each station we need to deduction the error (because its cumulative error). However, it's very small error and it remains always under tolerance.