Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The St. Johns River (Spanish: Río San Juan) is the longest river in the U.S. state of Florida and its most significant one for commercial and recreational use. At 310 miles (500 km) long, it flows north and winds through or borders twelve counties. The drop in elevation from headwaters to mouth is less than 30 feet (9 m); like most Florida waterways, the St. Johns has a very low flow rate 0.3 mph (0.13 m/s) and is often described as "lazy". Numerous lakes are formed by the river or flow into it, but as a river its widest point is nearly 3 miles (5 km) across. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County. The St. Johns drainage basin of 8,840 square miles (22,900 km2) includes some of Florida's major wetlands. It is separated into three major basins and two associated watersheds for Lake George and the Ocklawaha River, all managed by the St. Johns River Water Management District.
A variety of people have lived on or near the St. Johns, including Paleo-indians, Archaic people, Timucua, Mocama, French and Spanish settlers, Seminoles, slaves and freemen, Florida crackers, land developers, tourists and retirees. It has been the subject of William Bartram's journals, Marjorie Kinnan Rawlings' books, and Harriet Beecher Stowe's letters home. Although Florida was the location of the first permanent European settlement in what would become the United States, it was the last U.S. territory on the east coast to be developed; it remained an undeveloped frontier into the 20th century. When attention was turned to the state, however, much of the land was rapidly overdeveloped in a national zeal for progress. The St. Johns, like many Florida rivers, was altered to make way for agricultural and residential centers. It suffered severe pollution and human interference that has diminished the natural order of life in and around the river. In all, 3.5 million people live within the various watersheds that feed into the St. Johns River. The St. Johns, named one of 14 American Heritage Rivers in 1998, was number 6 on a list of America's Ten Most Endangered Rivers in 2008. Restoration efforts are under way for the basins around the St. Johns as Florida continues to deal with population increases in the river's vicinity.
What state remained an undeveloped frontier into the 20th century?