用所给单词的适当形式填空。

    Leonardo da Vinci lived in Italy in the fifteenth and sixteenth centuries. He was a student in Florence, where he studied painting and design. He began a lot of paintings, but he ____ (not finish) many of them. His picture of the Mona Lisa is ____ (famous) one in the world.

    Leonardo was interested in many things. He wanted ____ (know) about everything he saw. He thought the sun doesn't go around the earth. He wrote music. He designed a ____ (fly) machine 400 years before the first one flew. Many people didn't understand his ideas. It is difficult ____ (think) that one man can do so much.