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.

Problem: what is the sum of natural numbers between 30 and 100
Options: a. 4615, b. 4860, c. 5000, d. 5500, e. 5550
a

Problem: when jessica withdrew $ 200 from her bank account, her account balance decreased by 2 / 5. if she deposits an amount equal to 2 / 3 of the remaining balance, what will be the final balance in her bank account ?
Options: a. 300, b. 375, c. 400, d. 500, e. 575
d

Problem: a train 200 m long can cross an electric pole in 20 sec and then find the speed of the train ?
Options: a. 987, b. 271, c. 72, d. 27, e. 36

e