Two trains are moving at 50 kmph and 70 kmph in opposite directions. Their lengths are 170 m and 100 m respectively. The time they will take to pass each other completely is?  Choose the correct choice from the following answers.
A) 3 sec
B) 8 sec
C) 9 sec
D) 5 sec
E) 7 sec

Rationale: "70 + 50 = 120 * 5 / 18 = 100 / 3 mps d = 170 + 100 = 270 m t = 270 * 3 / 100 = 8 sec answer : b"
Answer: B