从句the earth goes around不缺少基本成分,所以如果我们自己填引导词的话应该填关系副词或“介词+which/whom”.没有跟the speed对应的关系副词,所以只能填“介词+which”,“以...速度”介词应该用at,所以从句就成了at which the earth goes around.
如果原句没有抄错的话,那只能把that当作关系副词.
that有时可作为关系副词,且可省略.
eg: This is the first time (that) I have been here.
I don't like the way (that / in which) he speaks to his mother.