As is known to all, the newly learnt words will soon be forgotten unless_____ used in everyday communication.

—What do you think of Zhang Liang Ying's English songs?—Very attractive and lively. Almost no other Super Girls can sing ______ at present, I think.  

I only intended to be there for a few days, but I         ended up staying for a whole month.

Although the country has had political independence for over a century,   it needs the support of its neighbors.

 _________, the subject belongs to biology.