幼虫阶段能引起皮肤损害的线虫有
A.丝虫B.似蚓蛔线虫C.旋毛形线虫D.钩虫E.毛首鞭形线虫
参考答案:D
已知圆方程x2+y2-2ax-4ay+5a2-4=0(a∈R).
(1)求圆的半径,圆心坐标并求出圆心坐标所满足的直线方程;
(2)试问:是否存在直线l,使对任意a∈R,直线l被圆截得的弦长均为2,若存在,求出直线l的方程;若不存在,说明理由.
--- ____ have you been a member of Greener China?
--- For two years.
A.How often
B.How long
C.When
D.How far