Because it lacks a main clause, "As the sun set" is known as what kind of sentence?

As the result of a 1614 wedding, who became John Rolfe's father-in-law? As the result of a 1999 merger, which company's trademarks include both a tiger and Pegasus? As the result of an injury in his first gunfight in 1876, which lawman began using a cane, often as a weapon? What was the seventh plague of Egypt in the Book of Exodus? In which gory 1985 horror film does Jeffrey Combs play Herbert West, who makes a serum that brings the dead back to lif… Which U.S. holiday was originally called Decoration Day? As the nation's rail capital in the 1850s, which city's population grew from about 30,000 to about 110,000? Which skater was world figure skating champion every year from 1927 to 1936?