Read the following paragraph and choose the topic sentence.

The brain is very sensitive to the level of energy in the light that passes through the eyes, with different colors of light carrying different amounts of energy. According to a recent research, blue suggests harmony and relaxation. It is the color of nature in relaxed form and it encourages social communication. Orange and yellow suggest a positive future, the point being that the color of signs surrounding roadwork is clearly meant to encourage frustrated drivers to think beyond the current delays. It is the same case with shapes. A triangular (三角的) sign has points and represents danger, which is why the shape is used for warning signs. Rectangular (矩形的）signs are the same shape as a book and therefore give information. Round signs are instructional. They look like the end of a pointing finger giving you an instruction.