Green is an important color in nature. It is the color of grass and the leaves on trees. It is also the color of most growing plants. Sometimes, the word "green" means young, fresh and growing. For example, a greenhorn is someone who has no experience. In the 15th century, a greenhorn was a young cow or an ox whose horns had not yet developed. A century later, a greenhorn was a soldier who had no experience in war. By the 18th century, a greenhorn had the meaning it has today--a person who is new in a job. Someone who is good at growing plants is said to have a green thumb . The expression comes from the early 20th century. A person with a green thumb can make plants grow quickly and well. Green is also the color used to describe the powerful feeling , jealousy . The green-eyed monster  is not a dangerous animal from outer space. It is an expression used about 400 years ago by the British writer William Shakespeare in his play "Othello". It describes the unpleasant feeling when someone has something he wants. For example, a young man may suffer from the green-eyed monster if you get a pay rise and he does not. In most places in the world, a green light means to move ahead. In everyday speech, a green light means agree to continue with a project. A greenhorn now refers to   _  .
a person who is new in a job a new soldier. a young horse a cow without horns