Why Was the Alaskan Pipeline Built Above Ground and Insulated

Why was the Alaskan pipeline built above ground and insulated?

A) to protect against damage from roots of large trees uprooted in windstorms
B) to keep the crude oil cool and highly fluid
C) to protect against damage from herds of elk and deer
D) to protect the pipeline against damage due to thawing ground and to keep the crude oil hot and fluid