Why does Bismarck feel it was necessary to avoid doing great harm to Austria in its war against Prussia?

Why does Bismarck feel it was necessary to avoid doing great harm to

Austria in its war against Prussia?
A. Damaging Austria too greatly would cause its leaders to join with
Prussia’s other enemies.
B. Austria was in control of the German city of Vienna, which had
once been a dependency of Prussia.
C. Both Austrians and Prussians were part of the German nation and
could be joined together without war.
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D. Russia and France had an alliance with Austria and would retaliate
against Prussia.