How many major tennis championships are there（）

A．mothers have the practice to go to work
B．working mothers have no time to stay with their children at home
C．mother’s position in a family has been raised
D．young wage-earners are economically independent

A．why the umbrella was so popular in Europe
B．when and how the umbrella was invented
C．the history and the use of umbrella
D．the development of the umbrella