在水面上撒上一些细炭黑粉，然后用一放大镜将阳光聚焦到水面上炭

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问题多项选择题

在水面上撒上一些细炭黑粉，然后用一放大镜将阳光聚焦到水面上炭黑粉的中央时，将会发现炭黑粉( )的现象。

A．(沉入水底

B．(保持不动

C．(向中央聚拢

D．(向四周扩散

答案

参考答案：D

单项选择题

有如下程序： #include＜iostream＞ using namespace std； int s=0； class sample {static int n； public：sample(int i){ n=i；}static void add(){ s+=n；} }； int sample：：s=0； int main() {sample a(2)，b(5)；sample：：add()；cout＜＜s＜＜endl；return 0； } 程序运行后的输出结果是

A．2

B．5

C．7

D．3

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