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 train travels at the rate of 10 miles / hr for the first hour of a trip , at 20 miles / hr for the second hour , at 30 miles / hr for the third hour and so on . how many hours will it take the train to complete a 210 - mile journey ? assume that the train makes no intermediate stops .
Options: a ) 6 , b ) 6.5 , c ) 7 , d ) 7.5 , e ) 5
A:
a