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').

[EX Q]: Pups remain in the den for about ____ weeks, and are cared for by the entire pack.\Question: Choose the right answer from options given a) ten b) three c) six d) no
[EX A]: c

[EX Q]: Female bilbies have a backward-opening pouch with ____ nipples.\Question: Choose the right answer from options given a) no b) two c) five d) eight
[EX A]: d

[EX Q]: Natural ytterbium is a mixture of ____ stable isotopes, with seven other unstable isotopes known.\Question: Choose the right answer from options given a) two b) seven c) six d) five
[EX A]:
b