Hummingbirds' Small Size Allows Them to Hover While Extracting Nectar

Hummingbirds' small size allows them to hover while extracting nectar from various flowers. This same small size requires large amounts of calories to maintain body temperature because

A) the breeze from their flapping wings constantly cools them off.
B) hummingbirds only eat at night.
C) they have a large body mass relative to their surface area.
D) hovering requires less energy than forward flight.
E) they have a large surface area relative to their body mass.