Wind, water, air, ice and heat all work to cause erosion. As the wind blows over the land, it often (1) ______ small grains of sand. When these grains of sand strike against solid rocks, the rocks are slowly worn away. In this way, even very hard rocks are worn away by the wind.
When particles of rocks or soil became loosened in any way, running water carries them down the (3) ______. Some rocks and soil particles are carried into streams and then into the sea.
Land that is covered with trees, grass and other plants wears away very slowly, and so loses very (4) ______ of its soil. The roots of plants help to (5) ______ the rocks and soil in place. Water that falls on grasslands runs away more slowly than water that falls on bare ground.
Điền vào số 4
A. aid
B. facilitate
C. assist
D. help
Đáp án D
Giải thích: help to do st: giúp đỡ
“Thus, forests and grasslands help to slow down erosion.” – Do đó, rừng và đồng cỏ giúp làm chậm lại sự xói mòn.