Problem: the circus sells two kinds of tickets : lower seats for $ 30 and upper seats for $ 20 . on a certain night , the circus sells 80 tickets and gets $ 2100 in revenue from the sales . how many tickets for lower seats did they sell ? 
Options: ['a ) 40', 'b ) 45', 'c ) 50', 'd ) 55', 'e ) 60']
c