Помогите с заданием  где поставила модальные глаголы не уверена что правильно  

Выберите нужную форму модального глагола: can (could),be able to, have to.

 

 

A FARMER AND HIS SONS

An old farmer was very ill. He was not happy, because his sons were always quarrelling.

He decided to teach them a lesson before he died. He said to one of them, “Try to break this bundle of sticks.” His soncan break the bundle. He asked all his other sons to do it, but they ... not break the bundle.

“Now untie it and try to break one of the sticks.” All the sons ... do it easily. “You could see now,” smiled the old farmer “that you have to stop quarrelling and help each other. Separated you are weak like the stick; untied you will always be strong.”

His son can  -правильно; but they couldnt break the bundle; all these son could do it остальное правильно

but they couldnt

can do it...