A farmer has 280 acres of land on which he plans to plant two different crops, crop A and crop B. For each acre of crop A he plants, his expenses are $25, and for each acre of crop B, his expenses are $30. The farmer has a limit of $10,000 for the expenses of planting the 280 acres. If a represents the acres of crop A and b represents the acre of crop B that the farmer plants, which of these represent the limits of a and b?