Presence of tail water in a gravity dam i) increases the principal stress ii) decreases the principal stress iii) increases the shear stress iv) decreases the shear stress The correct answer is A. (i) and (iii) B. (i) and (iv) C. (ii) and (iii) D. (ii) and (iv)

The correct answer is $\boxed{\text{(i) and (iii)}}$.

The presence of tail water in a gravity dam increases the hydrostatic pressure on the dam. This increases the vertical stress on the dam, which is a principal stress. The shear stress on the dam is not affected by the presence of tail water.

The principal stresses are the three maximum and minimum normal stresses acting on a point in a material.

The shear stresses are the stresses that act parallel to the surface of the material.

The vertical stress on the dam is the force per unit area acting perpendicular to the surface of the dam. The shear stress on the dam is the force per unit area acting parallel to the surface of the dam.

The hydrostatic pressure is the pressure exerted by a fluid at rest. It is equal to the weight of the fluid above the point in question divided by the area of the point.

The presence of tail water increases the hydrostatic pressure on the dam because the tail water is a fluid. The increased hydrostatic pressure increases the vertical stress on the dam, which is a principal stress. The shear stress on the dam is not affected by the presence of tail water.