要改掉（摆脱）年轻时候养成的习惯 很难.
To shake off the habit ( the one pick up when young ) is hard.
用排除法做很容易.
如果是that 或者 which ,这种情况通常是定语从句,那就把先行词代入从句,如果语义完整,那就没问题.很显然,这里the habit 才有可能是先行词,那句子就是 the habit pick up ,很显然成分不完整,要不就得是 the habit was picked up .如果是这个 which 跟 that 就都是正确答案了.
所以只能是 the one .