天然水及饮用水中主要含有六价硒或四价硒,水样加酸与不加酸保存对测定结果影响不大。
参考答案:对
Tony is twelve old, but his sister is only one old.
A.year; year
B.years; years
C.years; year
D.year; years
A.电流表的示数为3/11 A
B.通过灯泡的电流的频率为50Hz
C.电流表的示数为3/22 A
D.理想变压器原、副线圈匝数之比为2:1