Which two types of machine gun fire are classified with respect to the ground?

The classification of machine gun fire with respect to the ground is best captured by the terms grazing and plunging. Grazing fire occurs when the machine gun is positioned in such a way that the bullets travel just above the ground, maintaining a relatively flat trajectory. This type of fire is effective for targets that are close to the weapon, as it minimizes the risk of the projectiles hitting the ground before reaching the target.

On the other hand, plunging fire happens when the gun is elevated relative to the target. In this scenario, the bullets drop down at a steep angle, which is effective against targets that are further away. Plunging fire can cover a larger area and is particularly effective in engaging elevated or distant targets.

These two classifications are essential for understanding how to effectively employ machine gun fire on the battlefield, as they dictate the angles and ranges at which the machine gun can be used to maximum effect. The other options presented do describe different aspects of machine gun tactics, but they don't specifically categorize the types of fire in relation to the ground the way grazing and plunging do.