Julia prepares tax returns and does bookkeeping. last year her revenues from the tax and bookkeeping business were $150,000, and her expenses for the business were $15,000. when she started her tax and bookkeeping business, julia gave up her supplemental job doing in-home pet sitting. she used to earn $10,000 per year from pet sitting. assume that she incurred no costs for her pet sitting business. julia's implicit costs are