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: if a sum of money doubles itself in 20 years at simple interest, the ratepercent per annum is
Options: a. 12, b. 12.5, c. 13, d. 5, e. 14
d

Problem: two trains of length 100 meters and 200 meters are 450 meters apart. they are moving towards each other on parallel tracks, at speeds of 90 km / h and 72 km / h. after how many seconds will the trains meet ?
Options: a. 6, b. 8, c. 10, d. 12, e. 14
c

Problem: in a certain apartment building, there are one - bedroom and two - bedroom apartments. the rental prices of the apartment depend on a number of factors, but on average, two - bedroom apartments have higher rental prices than do one - bedroom apartments. let r be the average rental price for all apartments in the building. if r is $ 5,600 higher than the average rental price for all one - bedroom apartments, and if the average rental price for all two - bedroom apartments is $ 10,400 higher that r, then what percentage of apartments in the building are two - bedroom apartments ?
Options: a. 26 %, b. 35 %, c. 39 %, d. 42 %, e. 52 %

b