What is the primary function of the differential unit in the drive axle of an automobile?

1. To reduce the rotational speed of the drive wheels under all driving conditions.
2. To permit the outer drive wheel to rotate faster than the inner drive wheel when the vehicle is turning a corner.
3. To reverse the direction of rotation of the drive shafts when the reverse gear is engaged.
4. To lock the left and right axle shafts together when driving on slippery roads.