In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: if 3 taps are opened together a tank is filled in 3 hrs . one of the taps alone can fill it in 10 hrs . and another in 15 hrs . in how many hours will the third tap fill it ?
Options: a ) 7 hrs . , b ) 4 hrs . , c ) 5 hrs . , d ) 6 hrs . , e ) 8 hrs .
d