______ takes a lot of concentration and a great sense of balance to stay on it.
He is _________to staying up late and getting up late.
None of us was good at swimming, so the crossing of this dangerous river was really a_________.
Sometimes_________good knowledge of_________ first aid may mean the difference between life and death.
-- I will take part in the English lecture competition next week. -- ________.
. The glass doors have taken the place of the wooden ones at the entrance, ________ in the natural light during the day.