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 piece of work in 6 days and a alone can do it in 13 days . in how many days can b alone can do it ? 
Options: a ) 12 days , b ) 15 days , c ) 78 / 7 days , d ) 21 days , e ) 22 / 7 days
c

Problem: a train running at the speed of 120 km / hr crosses a pole in 18 seconds . what is the length of the train ? 
Options: a ) 560 , b ) 570 , c ) 580 , d ) 590 , e ) 600
e

Problem: the radius of the wheel of a bus is 100 cms and the speed of the bus is 66 km / h , then the r . p . m . ( revolutions per minutes ) of the wheel is 
Options: a ) 175 , b ) 250 , c ) 300 , d ) 330 , e ) none of these

a