I dont know his address,

I don't know his address, ______.

A．otherwise I will write to him

B．otherwise I would write to him

C．otherwise I had written to him

D．otherwise I would have written to him

B解析：在otherwise或or引导的并列分句中用虚拟语气。此时，otherwise或or引导的并列句相当于非真实性条件句中的主语部分，前一分句暗示了现在的情况，因而在 otherwise引导的分句中要用与现在事实相反的虚拟语气。