How did the end of the civil war railroad construction

It ended the labor shortage, as war veterans went to work on the railroad.

Explanation:

The Civil War caused an enormous labor shortage because when it started men that used to work, began to enlist. A report from the Secretary of War in 1862 said that there were 718,512 volunteers in the Union Army. This caused labor shortage, because men that could be working, were fighting.

When the War was over, despite the casualties, men were able to go back to work and projects were developed such as the railroad construction.

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering