下列哪种是血管源性肿瘤（）
A.毛细血管瘤
B.海绵状血管瘤
C.动脉瘤
D.血管肉瘤
E.室壁瘤
参考答案：A, B, D
— These farmers have been to the United States.
— Really? When ______ there? [ ]
A. will they go
B. did they go
C. do they go
D. have they gone
金属材料发生屈服时，为什么会产生吕德斯带？
