Given a sentence with a missing word, pick the answer option that best fills out the missing word in the sentence. Indicate each answer with its index ('a', 'b', 'c', 'd').

Example Input: Color codes are ____ digit combinations of letters and numbers that correspond to a specific color.\Question: Choose the right answer from options given a) six b) eight c) seven d) one
Example Output: a

Example Input: Some olive trees live to be ____ thousand years old.\Question: Choose the right answer from options given a) ten b) zero c) two d) seven
Example Output: c

Example Input: Negative numbers are left of ____ on a number line.\Question: Choose the right answer from options given a) seven b) six c) eight d) zero
Example Output:
d