Concept:
Soil erosion refers to the removal of top fertile soil by natural or human activities.
Step 1:
Floods and strong winds remove topsoil and therefore increase soil erosion.
\[
A \text{ is correct}
\]
Step 2:
Humidification does not directly increase soil erosion.
\[
B \text{ is incorrect}
\]
Step 3:
Overgrazing removes vegetation cover, exposing soil to erosion by wind and water.
\[
C \text{ is correct}
\]
Step 4:
Afforestation helps prevent soil erosion by binding soil with plant roots.
\[
D \text{ is incorrect}
\]
Step 5:
Thus, the correct factors are:
\[
A \text{ and } C \text{ Only}
\]
\[
\boxed{\text{(2) A and C Only}}
\]