Why did Zimmermann choose to send the message through the U. S. State Department?

Zimmermann choose to send the message through the U.S. State Department because he had np other option as the German U-boat was grounded because it needed repairs. He wanted to send the coded message to Count von Bernstorff in Washington, who at that time was the German foreign secretary, for negotiation with Mexico.

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(the answer is d)

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