My uncle ______ the club in 2014. He ______the club for over 3 years.
A:joined; has been a member of
B:joined; has joined
C:has joined; has joined
D:has joined; has been in
如图所示,烧杯中盛的是水,铁圈和银圈直接相连,在接头处用一根绝缘细丝吊住,并使之平衡,小心地从烧杯中央滴入CuSO4溶液,反应一段时间后,观察到的现象是( )
1000﹣1﹣3﹣5﹣7﹣9﹣…﹣37﹣39=( )