Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

When the first white men came to America, they found vast amounts of natural resources of tremendous value. Forests covered a large part of the nation; later gas, oil and minerals were found in unbelievable amounts. There was a great abundance of very fertile soil. Forests, prairies, streams and rivers abounded with wildlife. So vast were these resources that it seemed that they could never be used up. So forests were destroyed to make way for farmland. Grasslands and prairies were ploughed and harrowed. Minerals and oil were used in great quantities to supply a young industrial nation. Almost every river became the scene of factories, mills and power companies. Mammals and birds were slaughtered for food and sport.

Within a short time, the results were obvious. Floods caused millions of dollars worth of damage yearly. The very fertile soil was washed away or blown up in great clouds. The seemingly inexhaustible oil and minerals showed signs of depletion. Rivers were filled with silt from eroding farms and wastes from factories. Many of the rivers were made unfit for fish. Several species of birds disappeared, and some mammals seemed on the verge of going. Future timber shortages were predicted. In short, Americans soon became to realize that some sort of conservation program must be set up, if future as well as present Americans were to share in the resources that are the heritage of every American.

Much of the fertile soil of America ha__________ .

A. sunk deep into the earth                        

B. become the scene of factories

C. been covered by lakes                            

D. been eroded by wind and water