好发于老年人,常由残冠残根或不良修复体长期刺激引起的是()
A.褥疮性溃疡
B.Bednar溃疡
C.Riga-Fede溃疡
D.自伤性溃疡
E.化学灼伤性溃疡
参考答案:A
The Blacks are going to all over the world.
A.make
B.see
C.visit
D.travel
设随机变量X和Y的联合分布是正方形G=(z,y)|1≤x≤3,1≤y≤3上均匀分布,试求随机变量U=|X-Y|的概率密度p(u).