1． “Underground” is the only word in the English language which begins and ends with the letters “und”．
__which→that__
2．I will never forget the day when we spent in the old town with small houses.
__when→that/which或删去when__
3．He is unlikely to find the school which he taught 50 years ago.
__which→where__
4．I really like the photo of my family what my sister took in the city park last year.
__what→that/which__
5．We live in an age where more information is available with great ease than ever before.
__where→when__
