The correct answer is - temperature.
We see the stars on the night sky in multiple colors, and the variation in the colors of the stars comes from the differences in temperature. The cooler a star is, the more reddish it appears, the hotter the star is the more blueish or whitish they appear. From cool to hot, the stars can appear in the variations of red, orange, yellow, green, blue/white colors, and it is an easy indicator of the temperature of them.
Darwin is a great example. He created the theory of evolution, but because of the church ridiculing him, he couldn't come forth with his findings. Some parts of science may seem outlandish to society, and that can be very difficult to deal with.
Answer:
The respiratory and circulatory systems, for example, work together to provide oxygen to cells and remove carbon dioxide produced by the cells. The circulatory system takes oxygen from the lungs and delivers it to the tissues, then reverses the process with carbon dioxide.