Presence of tail water in a gravity dam_____________?

Presence of tail water in a gravity dam_____________?

1) increases the principal stress
2) decreases the principal stress
3) increases the shear stress
4) decreases the shear stress

The correct answer is
A. (1) and (3)
B. (1) and (4)
C. (2) and (3)
D. (2) and (4)