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').

Embrasures have ____ functions.\Question: Choose the right answer from options given a) three b) seven c) zero d) nine
a

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
b

Trees become susceptible to injury when they reach a height of about ____ feet.\Question: Choose the right answer from options given a) two b) ten c) eight d) three

d