Simplify the following text for a Grade 12 native English-speaking person.
Original: It symbolizes four of the world 's great rivers ( the Ganges , the Nile , the Danube , and the RÃ o de la Plata ) , representing the four continents ; the Americas counted as one , and Australia and Antarctica were unknown .
Simplified: Th art-work symbolizes four of the world 's great rivers ( the Ganges , the Nile , the Danube , and the RÃ o de la Plata ) , which is to represent the four continents ( Australia and Antarctica were unknown then ) .