附权益权证债券允许权证持有人（）。

A．以特定的价格即固定汇率，将一种货币兑换成另一种货币 

B．按约定条件向债券发行人购买黄金 

C．以约定价格购买发行人的普通股票 

D．以与主债券相同的价格和收益率向发行人购买额外的债券

Some people are bad at recalling names and faces, so it's possible that they have a condition called face blindness, ______ people suddenly have trouble recognizing family members, friends or even themselves．

A．for which

B．which

C．where

D．of which