Task: Question Answering
Input (English): Problem: a snail , climbing a 20 feet high wall , climbs up 8 feet on the first day but slides down 4 feet on the second . it climbs 8 feet on the third day and slides down again 4 feet on the fourth day . if this pattern continues , how many days will it take the snail to reach the top of the wall ? 
Options: a ) 7 , b ) 16 , c ) 17 , d ) 20 , e ) 21
Output (English): a