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: the present worth of rs. 2310 due years hence, the rate of interest being 15 % per annum, is :
Options: a. rs. 1440, b. rs. 1750, c. rs. 1680, d. rs. 1840, e. none of these
c

Problem: if a train, travelling at a speed of 90 kmph, crosses a pole in 4 sec, then the length of train is ?
Options: a. 281, b. 125, c. 288, d. 100, e. 121
d

Problem: annie and sam set out together on bicycles traveling at 15 and 12 km per hour respectively. after 40 minutes, annie stops to fix a flat tire. if it takes annie 30 minutes to fix the flat tire and sam continues to ride during this time, how many minutes will it take annie to catch up with sam assuming that annie resumes riding at 15 km per hour ?
Options: a. 20, b. 40, c. 60, d. 80, e. 100
d