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 Q]: Snowflakes are crystals that have many different shapes, but each snowflake has ____ points on it.\Question: Choose the right answer from options given a) six b) nine c) one d) seven
[EX A]: a

[EX Q]: Astronauts sleep only ____ hours each night, on average.\Question: Choose the right answer from options given a) six b) one c) four d) seven
[EX A]: a

[EX Q]: Ocular dryness responds to the use of artificial tears applied every ____ to three hours.\Question: Choose the right answer from options given a) no b) one c) three d) eight
[EX A]:
b