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: Atoms are extremely small, on the order of a few ____ billionths of a meter.\Question: Choose the right answer from options given a) ten b) seven c) nine d) eight
Example Output: a

Example Input: Big bluestem grows up to ____ feet tall.\Question: Choose the right answer from options given a) seven b) no c) nine d) eight
Example Output: c

Example Input: Healthy people can donate blood as often as once every ____ weeks.\Question: Choose the right answer from options given a) no b) five c) eight d) zero
Example Output:
c