"过去分词+as it is"是as结构的特定用法.The house was very quiet,isolated as it was on the side of a mountain.这个句子中,"as it is"(过去式用was)是强调"isolated",it 是主句中的主语,这里是"house".如果不要强调,则取消"as it is",句子就成为:"The house was very quiet,isolated on the side of a mountain".
这个结构也可以放到句首,变成:
Isolated as it was on the side of a mountain,the house was very quiet.
此题选过去分词才是唯一正确的选择.