设，证明：

（1）f（x）在其定义域内单调增加；

（2）

"I would almost rather see you dead." Robert S. Cassatt, a leading banker of Philadelphia, shouted when his twenty-year-old eldest daughter announced that she wanted to become an artist.
In the 19th century, playing at drawing or painting on dishes was all right for a young lady, but serious work in art was not. And when the young lady’s family ranked among the best of Philadelphia’s social families, such an idea could not even be considered.
That was how Mary Cassatt, born in 1844, began her straggle as an artist. She did not tremble before her father’s anger. Instead; she opposed him with courage and at last made him change his mind. Mary Cassatt gave up her social position and all thought of a husband and a family, which in those times was unthinkable for a young lady. In the end, after long years of hard work and perseverance, she became America’s most important woman artist and the internationally recognized leading woman painter of the time.

What do you know about Mary Cassatt’s marriage
[A] Her marriage failed because she never gave a thought to her husband and family.
[B] She never married because she did not want to be just a wife and mother.
[C] After marriage she decided to give up her husband rather than her career.