——Where did you met Tom?——It was on the farm-----his father

因为在定语从句中,像 where,when这种一般都等于“介词+ which/that”,如果还原之后句子中有介词的话,那么直接用that/which就可以了,如果没有介词,就应使用when/where或者prep+which/that.
在这道题中,将定语从句还原以后是his father worked the farm （前面on the farm的on是与met 连接的,met on the farm）,这样显然是不行的,缺少介词.所以应该使用where 或者on that
所以如果选A that的话问题应该是“ ——It was on the farm_____his father worked on.”
这样还原回去就是his father worked on the farm
不晓得你明白了没有?