How do you specify the class that will process the uploaded file?

A merchant requires the ability to configure contact information for their brick and mortar stores as a CSV file upload. The module already exists and contains an etc/adminhtml/system.xml file where the new field can be added.

How do you specify the class that will process the uploaded file?
A . <upload_model>MagentoConfigModelConfigUploadFile</upload_model>
B . <frontend_model>MagentoConfigModelConfigFrontendFile</frontend_model>
C . <backend_model>MagentoConfigModelConfigBackendFile</backend_model>
D . <source_model>MagentoConfigModelConfigSourceFile</source_model>

Answer: C