题目内容：
根据下列短文，回答题。He has influenced generations of artists but John Baldessari's own celebrity came relatively late. A physically imposing 79-year-old, he seemed slightly uncomfortable at a press conference at the Metropolitan Museum, where a travelling retrospective of his work has just opened for its final stop. Asked to distil his art for the many who have not heard of him, he responded cheerfully that it was not the job of an artist to "spoon-feed" viewers but to make them feel intelligent.
For decades Mr Baldessari has made art that challenges convention. Though his work is heavily conceptual, it is not designed to alienate--and is often very funny. In the wake of abstract expressionism, when painting was all, Mr Baldessari was investigating what it meant to make a painting, what the rules were, and how far he could stretch them. In the 1960s he created a series of works that featured mostly text on canvas, painted by sign professionals.
One, in black letters on canvas, reads "PURE BEAUTY". The words sit there like a taunt (嘲弄), a question, a declaration.
"I do not believe in screwing the bourgeoisie," Mr Baldessari explained in an interview. The irony in his work is not designed to reveal what is vacant in art, or what is silly about those who buy it. He just wants people to question what they are looking at. He pokes fun at the art establishment, but he lets viewers in on the joke. Art, he says, supplies"spiritual nourishment". Asked if a show at the Met sat uncomfortably with his subversive streak, Mr Baldessari did not miss a beat: "I would be happy to hang in a broom closet at the Met. It's a huge honour."
Mr Baldessari attributes some of his experimentation to having grown up in National City, California, a suburb just north of the Mexican border and well beyond the reach of any art scene. He was culturally isolated, but also free from the pressures of rejection. "I was trying to find out what was irreducibly art." His boldest early work was his "Cremation Project" in 1970, when he ceremonially burned nearly all the paintings he had made between 1953 and 1966. "I really think it's my best piece to date," he wrote of it at the time.
He supported himself by teaching, mainly at the progressive California Institute of the Arts in Valencia. He earned a reputation for being a revolutionary and generous teacher who inspired students to renounce painting and view art as something that happens in the brain. "Artists are indebted to him," said Marla Prather, who organised the show at the Met. He taught countless people how to make art from the ordinary stuff of life. Now the man himself is finally getting his due.
The main idea of this passage is ________. A．what the progress of Baldessari's art creating is
B．how Baldessari defines art
C．why Baldessari investigate the roles for art
D．how Baldessari became famous
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