David tunes pianos in his spare time for extra income. buyers of his service are willing to pay $135 per tuning. one particular week, david is willing to tune the first piano for $115, the second piano for $125, the third piano for $140, and the fourth piano for $175. assume david is rational in deciding how many pianos to tune. his producer surplus is

a) $-15.
b) $20.
c) $30.
d) $75.