A. Because my mother worked last night.

B. We finish class at 5 o'clock.     

C. I usually play sports with my friends.

①只要承担责任就一定会获得物质方面的回报

②承担责任往往伴随着获得回报的权利

③承担责任需要付出一定的代价

④承担责任，就要牺牲自我

请根据句意从方框中选择合适的单词，并用其适当形式填空，使句子通顺。

____ This shop sells all kinds of    ▲   from different countries.
____ I hear that her uncle has bought a new flat in the city centre    ▲   .
____ Jack won the    ▲   prize in the school reading contest and he felt very excited.
____ He had to make a    ▲   between staying with his parents and going abroad. 
____ You know Mike’s hobby is playing football, but    ▲   is collecting stamps.

在$△ABC$中，若$a=3,\cos A=-\frac{1}{2}$，则$△ABC$的外接圆半径是(     )