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'.

Q: Problem: a can do a piece of work in 4 hours ; b and c together can do it in 3 hours , which a and c together can do it in 2 hours . how long will b alone take to do it ? 
Options: a ) 15 hours , b ) 14 hours , c ) 12 hours , d ) 74 hours , e ) 79 hours

A: c
****
Q: Problem: two trains are moving in opposite directions with speed of 90 km / hr and 90 km / hr respectively . their lengths are 1.10 km and 0.9 km respectively . the slower train cross the faster train in - - - seconds 
Options: a ) 56 , b ) 40 , c ) 47 , d ) 26 , e ) 25

A: b
****
Q: Problem: if the population of a certain country increases at the rate of one person every 40 seconds , by how many persons does the population increase in 1 hour ? 
Options: a ) 90 , b ) 120 , c ) 150 , d ) 180 , e ) 160

A:
a
****