Question

So far Grandparents ______________________________ (保持联系) with their friends by writing to each other.

Answer

have kept in touch with

【解析】

句意：到目前为止，祖父母一直通过写信与朋友保持联系。

短语keep in touch with..和……保持联系；So far目前，谓语动词用现在完成时，其结构是have done的形式。keep的过去分词是kept。故答案为 have kept in touch with。