Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[Q]: Problem: an error 1 % in excess is made while measuring the side of a square. the percentage of error in the calculated area of the square is :
Options: a. 2.01, b. 30.1, c. 3.01, d. 25.01, e. 4.05
[A]: a

[Q]: Problem: the diagonal of a rectangle is 41 cm and its area is 20 sq. cm. the perimeter of the rectangle must be :
Options: ['a. 18', 'b. 28', 'c. 38', 'd. 48', 'e. 58']
[A]: a

[Q]: Problem: p, q and r together earn rs. 1710 in 9 days. p and r can earn rs. 600 in 5 days. q and r in 7 days can earn rs. 910. how much amount does r can earn per day ?
Options: a. s. 40, b. s. 60, c. s. 90, d. s. 100, e. s. 120
[A]:
b