Gustav Kramer Was Among the First to Determine That Birds

Multiple Choice

Gustav Kramer was among the first to determine that birds migrating during the day use the sun as a navigational cue by noting that caged starlings (Sturnus vulgaris) oriented in the normal migratory direction

A) as long as the sun was visible.
B) but only when they were released to fly normally.
C) unless the "sun" was being produced by a mirror.
D) when sky was overcast.