Task: Question Answering
Input (English): Problem: a and b can do a piece of work in 7 days , b and c in 8 days , c and a in 9 days . how long will c take to do it ? 
Options: a ) 14.1 days , b ) 18.8 days , c ) 21.4 days , d ) 24.3 days , e ) 26.5 days
Output (English): c