_____ on a clear day, far from the city crowds, the mountains gave him a sense of peace.

A．If walking

B．While walking

C．Walking

D．When he was walking

外周血反映骨髓幼红细胞增生程度的最可靠指标是（）

A.血红蛋白及红细胞计数

B.网织红细胞百分率

C.网织红细胞绝对值

D.出现有核红细胞

E.红细胞内出现Howelli-Jolly小体