The number of Americans applying for unemployment checks dropped last week to the lowest level since March, suggesting that most workers enjoy unusual job security. The Labor Department reported Thursday that jobless claims dropped by 9,000 to 211,000 last week. The four-week average of claims, which strips out week-to-week ups and downs, fell by 3,500 to 223,250. The overall numbers receiving unemployment benefits fell by 52,000 to 1.84 million.