A Pigeon First Completes a VR 20 Schedule on a Green

Multiple Choice

A pigeon first completes a VR 20 schedule on a green key and then a VI 30-sec schedule on a yellow key, following which it receives food. This behavior chain would have been best trained by starting with responding on

A) the green key.
B) the yellow key.
C) an orange key.
D) a greenish-yellow key.