Final answer:
The trade relationship with Germany caused distrust among the Brits and Russians due to Germany's influence on Iranian politics, which threatened the interests of the British and Russians in the region.
Step-by-step explanation:
The trade relationship with Germany caused distrust among the Brits and Russians due to Germany's influence on Iranian politics. Germany, under the leadership of Adolf Hitler, sought to strengthen its alliances with the Ottoman Empire and Austria-Hungary, both of which were declining empires. The British and Russians saw Germany's growing influence in Iran as a threat to their own interests in the region and therefore became distrustful of Germany.