In horizontal distance measurement, the basic formula for distance in stadia tachometry has an additive constant. An analytic lens is inserted in the tachometer to make this additive constant zero. This lens is

• A. Convex lens inserted between object glass and diaphragm
• B. Plano-convex lens between object glass and diaphragm
• C. Plano-convex lens between diaphragm and eye piece
• D. Convex lens inserted between diaphragm and eye piece