The heart, liver, and stomach are each examples of
a. an organ.
b. a tissue.
c. a nerve.
d. an organ system.

1. the is the thinnest part of the earth. 2. the is the thickest part of the earth. 3. what metals are mentioned in the text?

Environmental differences within ecosystems are generally caused by

Classify the structures as homologous or analogous, depending on their structure and function.