What was this structure intended to accomplish? A) to provide a trade route for merchants to move goods to Europe
B) to prevent invasion of nomads across the northern frontier of China
C) to end a series of civil wars between rival dynasties
D) to create a faster and more efficient way of traveling across the mountains

I think this is B

Explanation:

I was learning this in history

angel island immigration station was an immigration station located in san francisco bay which operated from january 21, 1910 to november 5, 1940,[4] where immigrants entering the united states were detained and interrogated. angel island is an island in san francisco bay. it is currently a state park administered by california state parks and a california historical landmark. the island was originally a fishing and hunting site for coastal miwok indians, then it was a haven for spanish explorer juan manuel de ayala. later, it was developed as a cattle ranch, then, starting with the civil war, the island served as a u.s. army post. during the island's immigration station period, the island held hundreds of thousands of immigrants, the majority from china, japan, india, mexico and the philippines. the detention facility was considered ideal because of its isolated location, making it very easy to control immigrants, contain outbreaks of disease, and enforce the new immigration laws.[5] the station is listed on the national register of historic places under the title angel island, u.s. immigration station, and is a national historic landmark. the station is open to the public as a museum – "a place for reflection and discovery of our shared history as a nation of immigrants”.

hope this