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').

Human females have ____ X chromosomes.\Question: Choose the right answer from options given a) two b) six c) one d) eight
a

Embryos develop and hatch in about four weeks, then spend four to ____ months in the tadpole stage.\Question: Choose the right answer from options given a) five b) nine c) eight d) two
a

Absolute pressure is based on a ____ reference point, the perfect vacuum.\Question: Choose the right answer from options given a) eight b) zero c) no d) three

b