Sam ______ his homework in his room now.[ ]
A. does
B. doing
C. is doing
D. do
答案:C
除去铜粉中混有少量的氧化铜,其主要操作过程是( )
A.在空气中燃烧
B.加适量稀硫酸、微热、过滤
C.加适量水、微热、过滤
D.加适量稀硫酸、微热、蒸发
()最容易被氧化。
A、亮煤
B、镜煤
C、暗煤
D、丝炭