With Respect to Modeling Earth's Climate System, Which of the Following

With respect to modeling Earth's climate system, which of the following best describes global circulation models (GCMs) ?

A) accurately simulate the past climate and are used to project the future climate
B) simulations of the past climate are too uncertain, thus future projections are fairly meaningless
C) are only models, hence cannot accurately simulate something as complicated as climate
D) can accurately simulate future temperature increases, but not precipitation
E) have too many unknown variables to be used with any degree of certainty