单项选择题
A.that farmers have lowered the yield of corn and soybeanB.that farmers have expanded croplandsC.that farmers have applied more chemical fertilizersD.that the top soil has been decreased greatly
A.120 years later.B.80 years later.C.50 years later.D.100 years later.
A.material from higher land is washed into themB.ditches and drains lead into themC.rivers become too swiftD.snow melts more rapidly nowadays