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').

Hummingbirds feed every ____ to fifteen minutes from sun-up to sundown.\Question: Choose the right answer from options given a) nine b) seven c) four d) ten
d

Diatoms look like ____ shells that fit together.\Question: Choose the right answer from options given a) zero b) one c) two d) seven
c

Normal humans have only ____ strands in a double-helix formation.\Question: Choose the right answer from options given a) two b) six c) zero d) no

a