Use the Following to Answer Question

Use the following to answer question:
-(Table: Variable Costs for Lots) Use Table: Variable Costs for Lots.During the winter,Alexa runs a snow-clearing service in a perfectly competitive industry.Assume that costs are constant in each interval;so,for example,the marginal cost of clearing each of the lots from 1 through 10 is $20.Also assume that she can only plow the quantities of the lots given in the table (and not numbers in between) .Her only fixed cost is $1,000 for a snowplow.Her variable costs include fuel,her time,and hot coffee.What is Alexa's shut-down price in the short run?

A) $20
B) $15
C) $50
D) $42