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 sum of all the integers a such that - 26 < a < 24 is
Options: a. 0, b. - 2, c. - 25, d. - 49, e. - 51
d

Problem: a sum of money at simple interest amounts to $ 645 in 3 years and to $ 690 in 4 years. the sum is :
Options: a. $ 153, b. $ 510, c. $ 598, d. $ 549, e. $ 675
b

Problem: gold is 19 times as heavy as water and copper is 9 times as heavy as water. in what ratio should these be mixed to get an alloy 15 times as heavy as water ?
Options: a. 3: 2, b. 3: 8, c. 5: 7, d. 3: 7, e. 3: 6

a