Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35.

     Flirting is a fundamental fixture in the sexual behavior repertoire, a time-honored way of signaling interest and attraction. It is a kind of silent language spoken by men and women around the world. The ways people communicate interest are so deeply rooted in human nature that the signals are automatically understood by all: from ways of glancing to movements such as licking one's lips, to meet nature's most basic command—find a good mate and multiply.

Flirting is not a trivial activity; it requires many skills: intellect, body language, creativity, empathy. At its best, flirting can be high art, whether the flirter is vying for a soul mate, manipulating a potential customer, or just being playful.

The process of flirting allows the signaling of interest to another in small increments, which is especially appealing to a partner. Flirting is driven by emotions and instinct rather than logical thought. Yet the gestures and movements used in flirting also provide reliable clues to a person's biological and psychological health.

Flirting has many parallels in the animal world, seen in the behavioral displays many animals engage in to signal not only their availability but their suitability. For example, penguins search for small pebbles to deliver to their partner of interest. Seahorses lock their tails together for a romantic swim. Bower birds use leaves, grass, and twigs to construct elaborate nests.

Across the animal kingdom, such actions are closely tied to seasonal reproductive readiness and signal reproductive fitness by one creature to another. Among humans, the repertoire of flirting behaviors can be deployed at will and the exact meaning of any gesture is usually a matter of interpretation.

(Source: https://www.psychologytoday.com/)

What is the main topic of the passage?