TASK DEFINITION: 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').
PROBLEM: Background radiation comes from ____ sources.\Question: Choose the right answer from options given a) zero b) three c) four d) no

SOLUTION: b

PROBLEM: Natural chlorine contains ____ isotopes.\Question: Choose the right answer from options given a) two b) six c) eight d) seven

SOLUTION: a

PROBLEM: Every earmark consists of a symbol in ____ positions .\Question: Choose the right answer from options given a) two b) eight c) ten d) seven

SOLUTION:
a