You should be able to get the return ticketed in an D class fare basis with the C-->B sector in an economy class fare bucket.
For example, the JFK-FRA UA D class fare allows you to use Y class if there is no D
Expertflyer:
Code:
BOOKING CODE EXCEPTIONS
VIA UA D REQUIRED WHEN AVAILABLE D- FARES WITHIN
USA
VIA UA A PERMITTED D- FARES WITHIN USA
VIA UA Y REQUIRED D- FARES WITHIN USA
VIA UA D REQUIRED WHEN AVAILABLE D- FARES BETWEEN
USA-CANADA
VIA UA A PERMITTED D- FARES BETWEEN USA-CANADA
VIA UA Y REQUIRED D- FARES BETWEEN USA-CANADA
VIA UA D REQUIRED WHEN AVAILABLE D- FARES WITHIN
AREA 2
VIA UA Y REQUIRED D- FARES WITHIN AREA 2
VIA UA D REQUIRED WHEN AVAILABLE D- FARES BETWEEN
AREA 2-AREA 3
VIA UA Y REQUIRED D- FARES BETWEEN AREA 2-AREA 3
VIA UA D REQUIRED D- FARES