The phenomenon by which a magnetic needle is deflected by the presence of masses of iron or steel called . . . . . . . .

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