question: What is a main practical difficulty in determining n ?
answer: the two most accurate methods , the watt balance and the X-ray crystal density method , do not appear to agree with one another
turker_answer: A main practical difficulty in determining n is that the two most accurate methods , the watt balance and the X-ray crystal density method , do not appear to agree with one another .
rule-based: A main practical difficulty in determining n is the two most accurate methods , the watt balance and the X-ray crystal density method , do not appear to agree with one another .