当主冷温差一定时,下塔压力随液氧纯度的降低而()。
A.升高
B.降低
C.不变
D.无法确定
参考答案:B
Can you make sure ______ the gold medal?
A.where Maria had put
B.where had Maria put
C.where has Maria put
D.where Maria has put
保存总铬的水样,则应加硝酸或硫酸至pH5。