Complex sentence: According to the Association of Science and Technology Centers, the Saint Louis Science Center is one of the top 5 science centers in the United States, and in 1991, was the most visited science center in the world.
Split into simpler sentences: According to the Association of Science and Technology Centers, the Saint Louis Science Center is one of the top 5 science centers in the United States. In 1991, it was the most visited science center in the world.