Question

German use of submarines to blockade britain during world war I led to
A german victory.
Trench warfare
A war of attrition
The united states entering the war.

Answer 1

I find the answer C?

Answer 2

The most likely answer to this questions is the U.S. entering the war, because German subs sank the U.S. ship Lusitania, which sparked the U.S. into participating in WW1.

Answer 3

I agree with esheehan^
Although you had a very good guess, a war of attrition has more to do with attacking resources and esteem to wear down the enemy. While a blockade could lead to this, the important thing is that it led to the U.S. joining the war.