Plants are very important. This is because plants can make food from air, water and sunlight. Animals and people can not make food from air, water and sunlight. Animals can get their food by eating plants and animals. People get their food by eating plants and animals, too. So animals and people need plants in order to live. This is why there are so many plants around us. There are two kinds of plants: flowering plants and non-flowering plants. You can know some trees from their flowers and fruits. Non-flowering plants do not grow flowers. You can not see many non-flowering plants around you. If you look carefully at the plants around us, you will find that there are many kinds of plants. Some plants are large and some are small. Most of them are green. Thanks to the plants around us we can live on the earth.

定值电阻R₁和R₂串联后，接在电压可调的电源两端，电路中的电流I随电源电压U变化关系的图像如图所示。已知R₁=4Ω，则R₂=____ Ω。

____÷4＝____，表示____里面有____个____。