104. Read the following passage, then answer the question below.

For centuries, ships traveling between the Atlantic and Pacific oceans had to sail around the southern tip of South America, a voyage of many thousands of kilometers. A canal across the narrow Isthmus of Panama promised to shorten the journey dramatically, but the obstacles were formidable. The French, led by Ferdinand de Lesseps, who had built the Suez Canal, began digging in 1881. Lesseps planned a sea-level canal like Suez, yet Panama was not Egypt. The route crossed mountains, and heavy rains caused landslides that buried the excavations. Worse, tropical diseases, especially yellow fever and malaria, killed workers by the thousands. The company went bankrupt in 1889.

The United States took over the project in 1904 and succeeded largely by changing the plan. Chief engineer John Stevens argued that a sea-level canal was impractical and proposed instead a system of locks that would lift ships to an artificial lake, Gatun Lake, about twenty-six meters above sea level, and lower them on the other side. Meanwhile, the physician William Gorgas, convinced by the work of Carlos Finlay and Walter Reed that mosquitoes carry yellow fever, organized campaigns to drain standing water and to screen houses. Yellow fever was soon eliminated in the canal zone, and malaria declined sharply. The canal opened in 1914.

The achievement carried costs that are easy to forget. Thousands of laborers, many from the Caribbean, died during construction, and the creation of the lake flooded villages and forests. In addition, the United States held control over the Canal Zone for decades, a source of tension with Panama that ended only when the canal was transferred to Panamanian control in 1999. Today, ships pass through an expanded set of locks completed in 2016, which can handle vessels too large for the original channel.

The word 'which' in paragraph 3 refers to