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

Sexual reproduction is varied in ____ ways.\Question: Choose the right answer from options given a) seven b) two c) five d) three
d

Tropical rainforests have up to ____ layers.\Question: Choose the right answer from options given a) four b) one c) no d) six

d