Answer: What introduced unification was Zollverein (German custom union) which eliminated customs among German states (principalities). Prussia was the most important actor whereas Austria was eliminated from Zollverein because of its highly protectionist attitude (Prince and Austrian kanzler von Metternich who defined Austrian policy till 1848 was against). Zollverein treaties were negotiated already in 1830s. Another factor was increasing importance (military and political) of Prussia that desired unification of Germany (at that time nonexistent). First significant Zollverein led to foundation of North German Confederation (1867) which was able to make economic agreements with its northern neighbours (Sweden and Norway). This was a political and economic context of German industrial revolution.
Explanation: Prussia did not have any significant rival among German states. Austria was already in decadence. When Metternich left Austrian court Austria was increasingly falling behind. Moreover Austria as a multinational state had a great disadvantage of growing nationalism.
