Answer: A. An introduction to the Gold Dust roller coaster.
The main purpose of the paragraph is to provide an introduction to the Gold Dust roller coaster. Although the passage mentions the average speed of the roller coaster, as well as the location of the park, these are supporting details, not the main idea. We know this because most of the information of the passage seems to support the idea of the Gold Dust roller coaster. Moreover, the passage ends with an invitation to try it out.