文档介绍：
laces during the flow of waterover the continents. Soluble ions such as calcium, sodium, potassium, and somemagnesium are dissolved and transported. Insoluble ions such as aluminum, iron,and silicon stay where they are and form the thin, fertile skin of soil on whichvegetation can grow. Sometimes soils are destroyed and transported mechanicallyduring flooding. The erosion of the continents thus results from two closelylinked and interdependent processes, chemical erosion and mechanical respective interactions and efficiency depend on different factors.
1. The word "modifying" in line 4 is closest in meaning to
(A) changing
(B) traveling
(C) describing
(D) destroying
2. The word "which" in line 5 refers to
(A) clouds
(B) oceans
(C) continents
(D) compounds
3. According to the passage , clouds are primarily formed by water
(A) precipitating onto the ground
(B) changing from a solid to a liquid state
(C) evaporating from the oceans
(D) being carried by wind
4. The passage suggests that the purpose of the "hydrographic network"(line 8) is to
(A) determine the size of molecules of water
(B) prevent soil erosion caused by flooding
(C) move water from the Earth's surface to the oceans
(D) regulate the rate of water flow from streams and rivers