You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
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: walking at 5 / 6 th of its usual speed a cab is 15 mnts late . find its usual time to cover the journey ? 
Options: a ) 25 m , b ) 45 m , c ) 32 m , d ) 75 m , e ) 62 m
Solution: d
Why? New speed = 5 / 6 th of usual speed new time = 6 / 5 th of usual time 6 / 5 ut - ut = 15 m ut / 5 = 15 m ut = 75 m answer is d

New input: Problem: if a , b and c together can finish a piece of work in 4 days . a alone in 12 days and b in 9 days , then c alone can do it in ? 
Options: a ) 11 , b ) 17 , c ) 18 , d ) 19 , e ) 16
Solution:
c