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: if the simple interest on a certain sum of money for 6 years is one – fifth of the sum, then the rate of interest per annum is
Options: a. 3.3 %, b. 7.2 %, c. 6.1 %, d. 5.0 %, e. 3.1 %
[A]: a


[Q]: Problem: what is the units digit of 32 ! + 50 ! + 4 ! + 4 ! ?
Options: a. 0, b. 2, c. 4, d. 6, e. 8
[A]: e


[Q]: Problem: a hollow iron pipe is 21 cm long and its external diameter is 8 cm. if the thickness of the pipe is 1 cm and iron weights 8 g / cm cube, then find the weight of the pipe.
Options: ['a. 3.696 kg', 'b. 3.686 kg', 'c. 2.696 kg', 'd. 2.686 kg', 'e. none of these']
[A]:
a