Which of the following system is used to working out the reduce levels of points from the staff readings taken in the field.

A. Temporary stations B. Main stations C. Subsidary station D. None

A. 16880m² B. 8440m² C. 1688m² D. 844m²

A. Cross staff B. Mason's square C. Theodolite D. Optical square

A. $$\sum {BS} + \sum {FS = {\text{First RL + Last RL}}} $$ B. $$\sum {FS} - \sum {BS} = {\text{Last RL + First RL}}$$ C. $$\sum {BS} - \sum {FS} = {\text{Last RL}} - {\text{ First RL}}$$ D. All