What was the main result of germany’s use of unrestricted submarine warfare during world war i? it limited the number of countries that became involved in the war. it weakened the german effort by overextending their resources. it push the united states into entering the war on the allied side. it convinced the allied powers that they could not win the war.

Context/detail:

Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania, which had happened in May, 1915.  When a German U-boat (submarine) sank the Lusitania, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship.

Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks.  But two years later, the Germans resumed such attacks. This inflamed public opinion against Germany once again and poised America to go to war.  

In addition, British intelligence intercepted and decoded a telegram (known as the "Zimmermann Telegram") that showed Germany was trying to secure Mexico as an ally against the United States.  The revelation of this plan inflamed feelings in the United States.  The telegram was sent in January, 1917.  In March, 1917, German Foreign Secretary Arthur Zimmermann admitted the telegram was, in fact, sent by his office.

In April, 1917, the US declared war on Germany in response to the Germans' submarine warfare and the Zimmermann Telegram.

yup! you are correct it is b.

he sent troops to protect the students as they entered the school.