WASHINGTON (AP) — U.S. authorities say they picked up nearly 19,000 children traveling alone across the Mexican border in March. It’s the largest monthly number ever recorded and a major test for President Joe Biden as he reverses many of his predecessor’s hardline immigration tactics. A complex mix of policies and conditions in the United States and Central America is driving the increase. It coincides with the Biden administration’s decision to exempt unaccompanied children from pandemic-related powers to immediately expel most people from the country without giving them an opportunity to seek asylum.