I think the country is a good place for the old man to --- .

不及物动词的不定式作定语修饰名词时,一般的需加上对应的介词.但若所修饰的名词为time, place或way时可以省去介词.
所以可以用（因为这两句中,动词live的不定式都是修饰places的,所以可以用to live in或to live作定语修饰）：
I think the country is a good place for the old man to live (in).
Zoos are terrible places for animals to live (in).
而I have never seen a zoo that was suitable for animals to live in.中to live in修饰的a zoo故不能省略介词in.