Which of these is a simple sentence?
immigrants who came to work in factories lived in urban neighborhoods.
“old” immigrants often settled on farms, but most “new” ones lived in cities.
in the past, many immigrants had come from northern and western europe.
immigrants worked long hours and faced crowded living conditions.

union blockade

explanation:

the union blockade in the american civil war was a naval strategy by the united states to prevent the confederacy from trading.

the blockade was proclaimed by president abraham lincoln in april 1861, and required the monitoring of 3,500 miles (5,600 km) of atlantic and gulf coastline, including 12 major ports, notably new orleans and mobile. those blockade runners fast enough to evade the union navy could only carry a small fraction of the supplies needed. they were operated largely by british citizens, making use of neutral ports such as havana, nassau and bermuda. the union commissioned around 500 ships, which destroyed or captured about 1,500 blockade runners over the course of the war.

travels along an axon. this triggers a wave of electrical activity that passes from the cell body along the length of the axon to the synapse