reading both verniers and taking the mean of the two

taking both face observations and taking the mean of the two

double sighting

taking mean of several readings distributed over different portions of the graduated circle

equal to true error

half the true error

two times the true error

four times the true error

error due to imperfect graduations

error due to eccentricity of verniers

error due to imperfect adjustment of plate levels

error due to line of collimation not being perpendicular to horizontal axis

error due to horizontal axis not being perpendicular to the vertical axis

index error i.e. error due to imperfect adjustment of the vertical circle vernier

error due to non-parallelism of the axis of telescope level and line of collimation

none of the above

increase

decrease

not change

either 'a' or 'b'