Chestnut blight is a fungus which caused the loss of all mature American chestnut trees in the United States. A number of scientists and members of the public are trying to bring this species back. Which of the following techniques has shown the most promise?

1. Removing exotic birds, such as starlings and sparrows, that transport the disease.

2. Infecting the chestnut blight fungus with a virus.

3. Planting American chestnuts outside of their former range.

4. Crossbreeding American chestnut with other chestnut species.