Mng giúp e với ạ Erosion of America's farmland by wind and water has been a problem since settlers first put the prairies and grasslands under the plough in the nineteenth century. By the 1930s, (46) __________ 282 million acres of farmland (47) __________ by erosion. After 40 years of (48) __________ efforts, soil erosion has accelerated (49) __________ new demands on the land by heavy crop production. In the years ahead, soil erosion and the pollution problems it causes are likely to replace petroleum scarcity as the nation's most critical (50)__________ resource problem. 46. A. more than B. more C. less D. than 47. A. damaged B. was damaged C. were damaged D. damages 48. A. conserve B. conservation C. conserving D. conservations 49. A. as B. since C. because D. due to 50. A. nature B. natural C. nation D. national