Andrea gives piano lessons. She has an opportunity cost of $50 per lesson and charges $60. Andrea has two students: Amy, who has a willingness to pay $70, and Carl, who has a willingness to pay $90. When the government puts a $20 tax on piano lessons and Andrea raises her price to $80, the deadweight loss is and the tax revenue is .

a $10, $20 b $10, $40 c $20, $20 d $20, $40