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:
Poles CAN occur at ____ frequency.\Question: Choose the right answer from options given a) six b) four c) zero d) eight

Ex Output:
c


Ex Input:
Nightmares happen frequently in childhood, especially between the ages of ____ and six.\Question: Choose the right answer from options given a) three b) eight c) no d) ten

Ex Output:
a


Ex Input:
Scientists recognize ____ major mass extinctions in the Earth's history.\Question: Choose the right answer from options given a) four b) five c) no d) zero

Ex Output:

b