___ himself in carrying out an experiment, he can’t spare time to look after his children. 

Which material can be thrown into the sea ________ on the nature of the material?

---How do you often spend your spare time? ---Personally speaking, there is nothing happier than reading a book, ____ a cup of tea.

The boys, ______ this frightening person, lost their tongues. 

Nankai University,_______ in 1919, has a history of more than 94 years.    (     )