WASHINGTON (AP) — A key long-term U.S. mortgage rate dipped this week, the second drop after a large increase two weeks ago.Mortgage company Freddie Mac said Thursday the nationwide average for a 30-year loan slipped to 4.19 percent from 4.20 percent last week. The average for a 15-year mortgage, a popular choice for people who are refinancing, was unchanged at 3.36 percent.The 30-year rate is down from 4.53 percent at the start of the year. Rates have fallen even though the Federal Reserve has been trimming its monthly bond purchases, which are intended to keep long-term borrowing rates low.