Bob took a photo of his girlfriend while she _________ at the party.
A:dances
B:was dancing
C:has danced
D:is dancing
某河水中含有的物质R在氧气的作用下发生反应,其反应的化学方程式为2R+2H2O+7O2 = 2FeSO4+2H2 SO4,则R的化学式是 ()