B 因为前面有了 portable computer 宾语已经提前了 所以 这里不能再出现it
如果后面加it就会重复,这是一个定语从句:Could you show me the portable computer (which) you want to have repaired
B 是错的应该是 A
you want to have repaired 定语修饰computer
have sth done= be done