On weekdays,we have to go to bed _______ nine o'clock. [ ]
A. in
B. by
C. on
D. at
答案:B
我厂热轧无缝钢管266系列设计规格范围最小直径为()mm。
能促进非血红素铁吸收的膳食因素是______。
A.磷酸
B.植酸盐
C.维生素C
D.动物肉类
E.有机酸