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.

[EX Q]: Problem: what is the units digit of ( 3 ^ 13 ) ( 4 ^ 13 ) ?
Options: a. 2, b. 4, c. 6, d. 7, e. 8
[EX A]: a

[EX Q]: Problem: for 2 consecutive yrs, my incomes are in the ratio of 4: 7 and expenses in the ratio of 3: 5. if my income in the 2 nd yr is rs. 42000 & my expenses in the first yr in rs. 23000, my total savings for the two - year is
Options: a. s. 8000, b. s. 9000, c. s. 9900, d. s. 9990, e. s. 10000
[EX A]: a

[EX Q]: Problem: a man rows his boat 75 km downstream and 45 km upstream, taking 5 hours each time. find the speed of the stream ?
Options: a. 6 kmph, b. 5 kmph, c. 2 kmph, d. 3 kmph, e. 1 kmph
[EX A]:
d