In 1990, festivities for the 20th anniversary of which annual observance included a trash cleanup trek on Mount Everest?

Who was British prime minister from 1990 to 1997? John Paxson, Horace Grant and Scottie Pippen helped which team win the 1991 NBA title, along with a certain star not me… In which 1990 film does Arnold Schwarzenegger play undercover detective John Kimble, forced to teach little kids? Which author's 1990 bestseller "Four Past Midnight" collected four novellas? Who won the Academy Award for Best Actress for the 1990 film "Misery"? Which film was the biggest U.S. box-office hit of 1990, ahead of "Ghost," "Dances with Wolves" and "Pretty Woman"? Jimmy Smits won a 1990 Emmy for playing attorney Victor Sifuentes on which legal drama? Which 1990 Madonna hit, named after a magazine, turned a dance from New York's ballroom scene into a craze?