Read the following article and select the best answer. Article: Li Shizhen was born in 1518. His father was a poor doctor. Li Shizhen often saw that people fell ill. He decided to study medicine so that he could be able to help people. Li Shizhen read many books about medicine. He found many of the old medical books to be full of mistakes. So his wish was to write a new one. He did his best to study medical science. He studied not only the herbs in his own garden, but also the wild ones. He set out many times on long journeys to collect herbs and talk with old farmers. He learned a lot from the working people. After many years of hard work and study, Li Shizhen finished his great work "Ben Cao Gang Mu" . At that time he was sixty. His book is now one of the greatest contributions of the Chinese people to the medical science of the world. Question: "Ben Cao Gang Mu" is_. - a medical book - a medical worker - a herb - a dictionary
a medical book