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.

[Q]: Question: A fish can go upstream at 35 kmph and downstream at 55 kmph, and then find the speed of the fish in still water?

 Option A: 45

 Option B: 55

 Option C: 35

 Option D: 65

 Option E: 70
[A]: Option A


[Q]: Question: The average age of students in section A of 100 students is 20 years and the average age of students in section B of 150 students is 30 years. Find the average age of students in both sections taken together?

 Option A: 26 years

 Option B: 30years

 Option C: 15years

 Option D: 25years

 Option E: 20years
[A]: Option A


[Q]: Question: If r > a and L < a, which of the following cannot be true?

 Option A:  d + L = 14

 Option B:  d - L = 7

 Option C:  d - L = 1

 Option D:  a - r = 9

 Option E:  a + d = 9
[A]:
Option D