One day a mother camel and her son walked through a desert. They were looking for water and grass. The son asked, "What do water and grass look like, Mum?"His mother answered, "Water looks like the sky, blue and clear.While grass is green, fresh and nice." After they walked for a day and a night, the son suddenly cried, "Look!Mum,there is water and grass over there!" "What a foolish talk,my boy!" The water and grass were on the left,while the desert was on the right.But because the mother camel was blind(   )in her left eye,she could only see a sea of yellow sand with her right eye.So she didn't believe what the young camel said. The next day they went on walking ,the young camel found a second _ ,but his mother still did not believe him. She said angrily, "No,that's not an oasis,but a desert. I have more knowledge and experience than you. You are wrong again. You should listen to me." This time the young camel was angry with his mother,too. So when he saw an oasis for the third time, he left his mother without telling her.He ran to the grass and ate his fill. But his poor mother was still walking on and she was still hungry and thirsty. How many times did the young camel find the oasis in total(  )?
Once Twice Three times Four times