30.(Some)seeds are viable,or capable of (growing)into healthy plants,for onl(a few)days after(fall) from the parent tree.
选D
31.Statistics (indicate)that(approximate)every 22years--within a range of 3 to 4 years--a(major)drought (occurs)in the United States.
选A,我觉得B应为approximately
39.(By)1810 the 23 towns of Hampshire County,Masschusetts,(had reached)a remarkable (uniform)of economic development(as well as) population density.
选D