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').

Female elephants have babies about ____ years apart, and they have only one each time.\Question: Choose the right answer from options given a) eight b) one c) zero d) three
d

Trains kill ____ people every year.\Question: Choose the right answer from options given a) zero b) three c) no d) four
d

Wind is renewable, and produces ____ emissions or by-products.\Question: Choose the right answer from options given a) five b) nine c) zero d) no

d