下列（）属于不良的土壤结构体 A.块状结构 B.核状结构 C.团粒结构 D.柱状结构

砧木要用（）苗，扦插容易、发育快、价格便宜的。

A.无性繁殖

B.扦插

C.压条繁殖

D.实生

If every student who was in the eleventh grade in 1996 and who used a computer in school at least once a week used a computer at least once a week in 1997, and if 20% of those students who were in the eleventh grade in 1996 and who did not use a computer in school at least once a week, did use a computer in school once a week in 1997, then what percent of twelfth-graders in 1997 used a computer in school at least once a week

A．(A) 40%

B．(B) 50%

C．(C) 60%

D．(D) 70%

E．(E) 80%

简述制订设计方案的主要内容。

下面（）不是PC-DOS的内部命令。

A.COPY

B.DISKCOPY

C.XCOPY

D.TYPE

废气涡轮增压器是将废气的能量转换成空气的压能和动能。