U.S. authorities caught more than 171,000 migrants at the U.S. border with Mexico in March, according to preliminary data shared with Reuters, the highest monthly total in two decades and the latest sign of the mounting humanitarian challenge confronting President Joe Biden. The total includes about 19,000 unaccompanied migrant children and 53,000 family members traveling together, the preliminary figures show. Single adults made up roughly 99,000 of the total. The Biden administration is struggling to find housing for unaccompanied children who have been backed up in crowded border stations and processing centers for days. The shelter system that houses the...