You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

[EX Q]: Question: If the operation ∆ is defined by a ∆ b = (b - a)^2/a^2 for all numbers a and b, and a ≠ 0, then what is the result when we evaluate this expression: (-5) ∆ (5 ∆ −5) ?

 Option A: 49/9

 Option B: 49/16

 Option C: 81/16

 Option D: 81/25

 Option E: 100/81
[EX A]: Option D

[EX Q]: Question: Two trains of equal are running on parallel lines in the same direction at 45 km/hr and 36 km/hr. The faster train passes the slower train in 36 sec. The length of each train is?

 Option A: 45

 Option B: 88

 Option C: 66

 Option D: 55

 Option E: 22
[EX A]: Option A

[EX Q]: Question: Gold is 19 times as heavy as water and copper is 9 times as heavy as water. In what ratio should these be mixed to get an alloy 15 times as heavy as water ?

 Option A: 3 : 2

 Option B: 3 : 8

 Option C: 5 : 7

 Option D: 3 : 7

 Option E: 3 : 6
[EX A]:
Option A