According to Billboard, what was Olivia Newton-John's biggest hit?

Physical is a song recorded by British-Australian singer Olivia Newton-John for her 1981 eleventh studio album of the same name. It was released as the album's lead single on 28 September 1981. "Physical" was an immediate smash hit, shipping two million copies in the United States, where it was certified Platinum by the Recording Industry Association of America (RIAA), and spent 10 weeks at number one on the Billboard Hot 100. "Physical" ultimately became Newton-John's biggest hit and cemented her legacy as a pop superstar.

Source: Wikipedia