图示结构为（  ）体系。
A、几何不变无多余约束
B、几何不变有多余约束
C、瞬变
D、常变

单项选择题Regarding to the polarization, which statement as follows is wrong? A、The polarization is used to indicate the degree of polarization of dielectric. B、If is the vector sum of dipole moments in a volume , we can obtain . C、The unit of polarization is . D、The polarization is the dipole moment per unit volume.

单项选择题如图所示：镜下可见血栓被肉芽组织取代，血栓中可见大小不等的不规则腔隙，部分腔隙内已有血管内皮细胞覆盖，腔内可见红细胞，这种现象称为（ ）。A.血栓硬化B.侧支循环形成C.血栓机化与再通D.血栓脱落