Bob goes to the Children's Palace every

.It is said that the early European playing-cards ______for entertainment and education.

A．were being designed

B．have designed

C．have been designed

D．were designed

反应体内贮铁情况的较敏感的指标是

A．血清铁蛋白(S＜12μg/L

B．红细胞游离原卟啉(FE＞0.9μmol/L

C．血清铁(S1)＜9.0～10.7μmol/L

D．总铁结合力(TIB＞62.7μmol/L

E．转铁蛋白饱和度(T＜15％

He had to retire(退休) early ______ poor health.

A．as a result

B．because

C．so

D．because of

以下哪一个网站的商业模式是属于BTOB电子商务模式（）。

A．Amazon

B．凡客诚品

C．alibaba

D．sina

与辐射有光的物理量有（）

A.日地距离

B.光通量

C.光通量密度

D.照度