恶性肿瘤分类中，“来源于间叶组织的恶性肿瘤”属于（）
A.癌
B.肉瘤
C.白血病
D.母细胞瘤
E.霍奇金病
参考答案：B
—You haven't been to the West Lake, have you?
—_____. But I will go there with my parents this summer vacation. [ ]
A. No, I haven't
B. No, I didn't
C. Yes, I have
D. yes, I did
纯化系统的工作周期通常定为（）。
