The train started moving. There were many people of all ages. Most of them were working men and women and young college boys and girls. Near the window sat an old man with his son, who looked about thirty years old. As the train moved, the son was filled with joy  because he was very excited at the scenery outside. He said in a high voice, "See, Dad, the green trees and the scenery are very beautiful!" It made the other people feel very strange. Everyone started talking in a low voice about the old man's son. A few minutes later it started raining. Raindrops fell on the travelers through the opened window. Seeing the rain, the old man's son said happily, "See ,Dad, how beautiful the rain is!" Hearing these words, a young lady became angry with the man, as the raindrops were dropping on her new clothes. She shouted at the noisy son and his father. "Can't you see it is raining? You! Old man! If your son is not feeling well, get him to a mental  hospital quickly and please don't disturb us any more!" The old man's face turned red and then he answered in a low voice, "We are on the way back from hospital. He couldn't see anything at birth. Only last week he could see again. The rain and the nature are new to him. Please _ him." All the other people became quiet. How old was the old man's son?
About 7 years old. About 20 years old. About 50 years old. About 30 years old.