S-hydrograph is used to obtain unit hydrograph of A. shorter duration from longer duration B. longer duration from shorter duration C. both (A) and (B) D. none of the above

The correct answer is: C. both (A) and (B).

An S-hydrograph is a type of hydrograph that is used to obtain a unit hydrograph of a shorter duration from a longer duration hydrograph. The S-hydrograph is constructed by first plotting the original hydrograph on a semi-logarithmic graph. The original hydrograph is then shifted to the right until the peak of the S-hydrograph coincides with the peak of the original hydrograph. The S-hydrograph is then scaled so that the area under the S-hydrograph is equal to the area under the original hydrograph. The resulting S-hydrograph can then be used to obtain a unit hydrograph of a shorter duration by multiplying the S-hydrograph by the desired duration.

The S-hydrograph can also be used to obtain a unit hydrograph of a longer duration from a shorter duration hydrograph. To do this, the S-hydrograph is first plotted on a semi-logarithmic graph. The S-hydrograph is then shifted to the left until the peak of the S-hydrograph coincides with the peak of the original hydrograph. The S-hydrograph is then scaled so that the area under the S-hydrograph is equal to the area under the original hydrograph. The resulting S-hydrograph can then be used to obtain a unit hydrograph of a longer duration by dividing the S-hydrograph by the desired duration.