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').

Nodes are points in space that have ____ probability for locating an electron there.\Question: Choose the right answer from options given a) no b) six c) zero d) three
c

Some European countries require ____ years of education.\Question: Choose the right answer from options given a) no b) nine c) six d) three
d

Black rhinos have ____ horns, one longer than the other.\Question: Choose the right answer from options given a) no b) two c) seven d) five
b