The bus company has informed us that there is no bus available for the date and time we requested for the school field trip. To solve this problem, the principal proposes that we send out a parent volunteer sheet to request drivers for that day. If not enough drivers are found, we may have to call a private transportation company. We hope to get enough volunteer drivers, however, because hiring a transportation company would be very costly for families. What is the problem stated in the text?

What is the primary solution the author proposes for the problem?