Soil is a rock that is broken by weathering and changes in physical and chemical properties to form a loosely structured weathering crust. The weathering crust is a surface material that has formed over a long time under the action of climate and biology. It is called soil invader. According to the above definition, which of the following is soil invaders? ['Massive gravel in the Gobi Desert.', 'Plenty of pebbles present on the river bed.', 'Bricks excavated during archeological excavations.', 'Weeds growing in rock cracks.']