Do you know Edison wore many caps during his life? First he was just a poor working boy. He wore work caps of many kinds. Later in life he became a famous inventor. Then he wore the caps of many famous schools and colleges. But his most important cap was his thinking cap. When he was a young boy, Edison's thinking cap saved his mother's life. One night, she suddenly felt ill. A doctor was called in. He had to operate(做手术) on her at once, but there was not enough light in the room. Edison quickly put on his thinking cap. He got all the oil lights in the house and put them on a long table. Then he placed a big mirror behind them. At once the room was quite bright. The doctor could operate, and his mother was saved. Edison was a thinking man. Yet, he was a working man and doing man, too. He was good with his hands and his head. Edison's inventions have changed the world greatly. Today, big trains run on electricity. Electricity lights light up the streets of towns and villages. Science uses it every day. Records, shining lights, moving pictures and ringing telephones are very common now. Edison's thinking cap made them. Usually, caps keep the head warm. But thinking caps keep the world warm.

When the doctor was to operate, the room was dark because there was no ________.

A.windows

B.caps

C.enough lights

D.fire