Given the task definition and input, reply with output. 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: a and b together can do a work in 6 days . if a alone can do it in 15 days . in how many days can b alone do it ? 
Options: a ) 10 , b ) 99 , c ) 77 , d ) 55 , e ) 21

a