Do you like eating chocolate? Companies spend millions of dollars on tools and machines to make chocolate. With a little ingredients, however, you can turn your kitchen into a chocolate factory. Today we will teach the Cocoa Powder （可可粉）Method（方法）, it is easier for beginners. Try it and make your own homemade chocolate.
Here are the ingredients (原料) you need:
cocoa powder — 2 cups (220g) ; butter — 3/4 cup (170g); sugar — 3/4 cup (100g); milk — 2/3 cup (150ml); salt — a little bit ; water — 1 cup (235ml).
      Now, please follow these steps. First, mix the cocoa powder and butter in a bowl until you have a smooth paste (酱). Then , add the cocoa powder mixture (混合物)  to the hot water and stir（搅拌）. Allow the temperature to rise back up. Put the hot mixture into a bowl,  stir the sugar mixture into the hot cocoa mix. Add milk, stir until smooth. At last, put the mixture into different containers（容器）. The chocolate will take the shape of the container. You can put it in the fridge to harden（使变硬）it.
We hope you can enjoy your homemade chocolate.

善良要付诸行动，表现在（  ）

①对需要帮助的人给予力所能及的帮助    ②关怀和体贴别人，为别人的需要作出贡献

③体验生命的快乐                                        ④为了得到某种回报，去做一些看上去善良的事

一列简谐横波沿 $x$ 轴正方向传播， $t=0$ 时刻的波形如图所示（此时波恰好传播到 $x=6m$ 处）。质点a平衡位置的坐标 $x_a=2.5m$ ，该质点在 $8s$ 内完成了4次全振动，则该列波的波速是____ $m/s$ ；位于 $x=20m$ 处的质点再经____ $s$ 将第一次经过平衡位置向 $y$ 轴负方向运动。