在生物圈中,有的生物只有一个细胞,如草履虫,这类生物被称为____________;大多数生物都是多个细胞构成的,这类生物称为______________。
单细胞生物 多细胞生物
It ________ we had stayed together for a couple of weeks _________ I found we had a lot in common. [ ]
A. was until; when
B. was until; that
C. wasn't until; when
D. wasn't until; that
在X线摄影中,光电效应的优势是()
A.产生有效的散射
B.对胶片产生灰雾
C.增加X线对比度
D.减少曝光时间
E.增加辐射剂量