--- _________ does your cousin go to the gym?                                                                                     --- Twice a week.                                                                                                      

A．How long

B．How often

C．How soon

D．How far

过氧化钠( Na₂O₂)可与空气中的二氧化碳反应放出氧气，常用在缺乏氧气的场合，如潜水、航天等方面。通常钠没有可变化合价，则Na₂O₂中氧元素的化合价为  [ ]

A．0    

B．-1    

C．-2    

D．+2