For the reaction, CaCO3 (s)<---> CaO (s) + CO2 (g) the expression for
the equilibrium constant is:
a. [CaO][CO2]
b. [CO2]
c. [Ca][CO2]/[CaCO3]

Hi again teatea!
Since pure solids are not included in the equilibrium constant, and
since CaCO3 and CaO are in solid state, the correct answer is then
(b): [CO2]. Check the following page to see the rules involved in
writing the equilibrium constant:

Learning Chemistry Tutorials
http://www.learnchem.net/tutorials/eq.shtml


Google search strategy:
equilibrium constant
://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=equilibrium+constant


Best wishes!
elmarto

Actually, pure solids are considered in the equilibrium equation, only
their contributions are multiplicative by their acitivity, which for a
pure substance is 1.

Equilibrium expressions only use concentration as an approximation for
activity or fugacity, which would be the correct thermodynamic
quantities.  Since the activity of a pure substance is 1, they only
"seem" to have no contribution.