Simplify the following text for a Grade 6 native English-speaking person.
Original: Saint Croix is an island in the Caribbean Sea , and a county and constituent district of the United States Virgin Islands ( USVI ) , an unincorporated territory of the United States .
Simplified: Saint Croix ; is an island in the Caribbean Sea . It is a county and a district of the United States Virgin Islands , an a territory of the United States .