圆$C_1$：${\left(x-1\right)}^2+{\left(y+2\right)}^2=16$与圆$C_2$：${\left(x-2\right)}^2+y^2=1$的位置关系是(    )

个经济的制约作用已被越来越多的人所认识，打破这种体制的要求已非常强烈，而这种体制

也确实到了非改不可的时候了。”这可以印证戈尔巴乔夫改革（  ）

选择括号内恰当的单词或词组填入空白处，使句意完整、正确。 
Are you going to____ （join / take part in）the school rowing club?
I think the computer is ____（used / useful）.
I spent two hours____ （sweep / sweeping）the house yesterday.
The house is so expensive that I can't____ （share / afford）it.
Even scientists aren't sure ____（if / whether）or not there are UFOs.