我们所居住的地球是太阳系第三颗行星。 the planet on which we are living这部分是介词on+关系代词which引导的________从句，其中which（能，不能）换成that。 … the earth, the planet on which we are living ,…=…the earth, the planet _________________we are living on,… 拓展：在介词+关系代词这种结构中，介词的选择从两方面考虑。一，与先行词的搭配关系；二，与从句位于动词的搭配。如 the day _____ which i saw him was the greatest of my life. there wasn’t a single person on the dark street _______ whom she could turn to help.
