Question :
A sentence is given in which certain words are highlighted. Choose the option with the pair(s) of words that need to be interchanged to make the sentence grammatically and contextually correct. If no change is required, mark 'E' as your answer.
Mathematicians have spent millennia (A) trying to understand prime numbers - those only divisible by themselves and 1 - but so far no one has discovered (B) a pattern for the primes, but that changes (C) now as mathematician Simon Plouffe has discovered a new method to produce long sequences of prime numbers, improving on previous (D) efforts.
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