What is a source of energy that is internal to the earth systems?
A. Sun
B. Moon
C. Tides
D. Radioactive decay
E. All of the above

Ascientist wants to know how individual lions within a pride interact with each other in their own environment. to do this, the scientist sedates and tags all of the lions within a pride. then, he places several remotely-controlled video cameras near the lions' den and performs an observational field study. he collects continuous video footage over the span of one year, analyzes the video, and then forms conclusions based on his observations.