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').

Ex Input:
Natural indium contains ____ isotopes.\Question: Choose the right answer from options given a) one b) eight c) two d) seven

Ex Output:
c

Ex Input:
Genetic mapping divides the chromosomes in ____ bands.\Question: Choose the right answer from options given a) ten b) two c) four d) nine

Ex Output:
b

Ex Input:
Coastal kelp beds, for example, appear to be three times larger than in the past four or ____ years.\Question: Choose the right answer from options given a) five b) two c) three d) ten

Ex Output:
a