Choose the meaning which best fits the underlined phrase from the passage. Closely guarded secret

In 1973 a Japanese sericulturist arrives in Malawi with a batch of 40,000 silkworm eggs. They were taken to the Bvumbwe Agricultural Research Station in Thyolo District. In this station, work is being done to determine favourable silkworm rearing conditions and areas where mulberry trees, leaves the worms feed on, could grow well. According to researchers, the silkworms- which eventually develop into cocoons from which raw silk is produced – do well with warm climatic conditions.
Silk is one of the strongest of fibers. Infect, for thousands of years, silk fabrics have been regarded as the most beautiful and durable materials woven by man. Many people call silk the cloth of kings and queens’.
The weaving of silk originated in china. An old Chinese book believed to be written by Confucius, tells us that the wife of Emperor Huang ti was the first person fabrics of silk. Around 2640 B.C Emperor Huang ti asked his wife His Ling-shih to study the worms that were destroying the mulberry trees in his garden. The Empress took some of the cocoons into the palace to see what they were made of. She dropped one of the cocoons into a bowl of boiling water and was amazed to see a cobweb-like tangle separate itself from the cocoon. She picked up the gauzy mass and found that one of the threads could be unwound almost without end from the cocoon. His Ling-shih had discovered silk. She was delighted with the discovery and even wove a ceremonial robe for the Emperor out of the cocoon threads. After that, the officials in the Emperor’s court wore brightly dyed silk robes on important occasions.
People in other countries regarded the new fibers as something rare and beautiful. A few traders went to China to learn about making cloth from silk, but the Chines kept their silkworm a closely guarded secret.