Which war invention led to the United States joining
WWI?

That would be the Zimmerman Telegram.

Explanation: The Zimmermann Telegram galvanized American public opinion against Germany once and for all. The telegram was considered perhaps Britain’s greatest intelligence coup of World War I and, coupled with American outrage over Germany’s resumption of unrestricted submarine warfare, was the tipping point persuading the U.S. to join the war.

D, because civic virtue is basically doing what’s morally good.