Passage: The main political parties of the 19th century were the Catholic Party and the Liberal Party, with the Belgian Labour Party emerging towards the end of the 19th century. French was originally the single official language adopted by the nobility and the bourgeoisie. It progressively lost its overall importance as Dutch became recognized as well. This recognition became official in 1898 and in 1967 the parliament accepted a Dutch version of the Constitution.The Berlin Conference of 1885 ceded control of the Congo Free State to King Leopold II as his private possession. From around 1900 there was growing international concern for the extreme and savage treatment of the Congolese population under Leopold II, for whom the Congo was primarily a source of revenue from ivory and rubber production. Many Congolese were killed by Leopold's agents for failing to meet production quotas for ivory and rubber. It is estimated that nearly 10 million were killed during the Leopold period. In 1908, this outcry led the Belgian state to assume responsibility for the government of the colony, henceforth called the Belgian Congo. A Belgian commission in 1919 estimated that Congo's population was half what it was in 1879.Germany invaded Belgium in August 1914 as part of the Schlieffen Plan to attack France, and much of the Western Front fighting of World War I occurred in western parts of the country. The opening months of the war were known as the Rape of Belgium due to German excesses. Belgium assumed control of the German colonies of Ruanda-Urundi (modern-day Rwanda and Burundi) during the war, and in 1924 the League of Nations mandated them to Belgium. In the aftermath of the First World War, Belgium annexed the Prussian districts of Eupen and Malmedy in 1925, thereby causing the presence of a German-speaking minority.

Question: Why did the Dutch king value possession of the Congo?
a source of revenue from ivory and rubber production