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: Japan is a highly urbanised country, with approximately ____ in every four people living in cities.\Question: Choose the right answer from options given a) ten b) no c) five d) three
Example Output: d

Example Input: Some male fish return after just ____ years.\Question: Choose the right answer from options given a) no b) two c) one d) six
Example Output: b

Example Input: Mammals fall into ____ groups that differ in the way they use the amniotic egg.\Question: Choose the right answer from options given a) three b) no c) five d) six
Example Output:
a