单项选择题

Sigmund Freud was a world-famous doctor of Vienna(维也纳). He was also a man full of humour(幽默). Once, at his 70th birthday party, a friend asked Freud if he could put his work into several words. "Well," said Freud, "we take the sick out of their trouble and return them to the common suffering. "
As a devoted(忠实的) son, Freud often visited his old mother. His mother usually spent her summers in a small mountain town in Austria(奥地利). The King Josef usually spent his summers there, too. One day in the summer, a band(乐队) was playing a lively tune(曲子) when Freud’s mother was sitting at the window and watching the people singing and dancing on Main Street. The old woman had a poor memory. She heard the band playing, but forgot it was the King’s birthday. Freud was visiting his mother on her 95th birthday. He told his mother, "Mama, the band is playing for your birthday. " She believed him and had a wonderful day.
Freud’s mother felt very happy on her 95th birthday because ______.