You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

Question: A is twice as fast as B. If B alone can do a piece of work in 18 days, in what time can A and B together complete the work?

 Option A: 10

 Option B: 16

 Option C: 18

 Option D: 6

 Option E: 12
Option D

Question: 3 + 3 + 3 + 2 × 3^2 + 2 × 3^3 + 2 × 3^4 + 2 × 3^5 + 2 × 3^6 + 2 × 3^7 =

 Option A: 3^7

 Option B: 3^8

 Option C: 3^14

 Option D: 3^28

 Option E: 3^30
Option B

Question: A palindrome is a number that reads the same forward and backward, such as 121. How many odd, 4-digit numbers are palindromes?

 Option A: 40

 Option B: 45

 Option C: 50

 Option D: 90

 Option E: 2500

Option C