砒霜是三氧化二砷（As₂O₃）的俗称，有剧毒，As₂O₃中As的化合价为（　　）

A．-3

B．-2

C．+2

D．+3

A Bluetooth device can be either a master or a slave and any of the devices within a （71） can be the master．There is only one master and there can be up to （72） active slave devices at a time within a single network．In addition, a device may be a standby slave or a parked slave．There can be up to （73） parked slaves．If there are already maximum number of active slaves, then a parked slave must wait until one of the active slaves switches to （74） mode before it can become active．Within a network, all （75） communications are prohibited.

空白处（72）应选择（）

A．7

B．15

C．63

D．255