单项选择题
A．the butcher found himself cheated by the smart animal B．the dog was clever enough to write on the paper C．the dog dared not go to tile butcher's any more D．the butcher was told not to give any meat to the dog
A．some different comets appearing several times B．the same comet appearing at different time C．three different comets appearing three times D．several comets appearing at the same time
A．by doing experiments B．by means of his own careful observation C．by using the work of other scientists D．by chance