I want to know the answer to the following question:
What milestone did Celine Dion, Mariah Carey, Phil Collins, Michael Bolton, Whitney Houston and Shania Twain share on the 1990s adult contemporary charts?
All the background I'm provided with is that "In addition to Celine Dion, who has had significant success on this chart, other artists with multiple number ones on the AC chart in the 1990s include Mariah Carey, Phil Collins, Michael Bolton, Whitney Houston and Shania Twain.". Is that enough to answer the question?