In Guns, Germs, and Steel, using the chart on page 87, which of the five proximate factors is the most important product of an agricultural society?

In my opinion, the proximate factor that is the most
important product of an agricultural society is epidemic diseases.  The others are
things that come from agriculture, but not so directly.  You can see this fact in the
chart itself.

The presence of epidemic diseases comes
directly from the presence of many domesticated animal species in the society.  Germs
from the animals cross over to humans, causing epidemic diseases to
arise.

The other proximate causes are important, but they
are not so directly tied to agriculture.  For those, there are many steps between
agriculture and the proximate causes.