Helen has two sisters and a brother. Her brother is working, but _________her sisters are still at school.

Mum couldn’t find any help and had to do the housework_________.

— _______ can a tiger weigh, do you know? — Yes, I know that a big one can weigh over 300 kilos. 

The little boy can't find        mother, so he is crying.

比较划线部分的读音，选出一个与其他三个发音不同的单词.