Article: Liberia has the highest ratio of foreign direct investment to GDP in the world, with US$16 billion in investment since 2006. Following the inauguration of the Sirleaf administration in 2006, Liberia signed several multibillion-dollar concession agreements in the iron ore and palm oil industries with numerous multinational corporations, including BHP Billiton, ArcelorMittal, and Sime Darby. Especially palm oil companies like Sime Darby (Malaysia) and Golden Veroleum (USA) are being accused by critics of the destruction of livelihoods and the displacement of local communities, enabled through government concessions. The Firestone Tire and Rubber Company has operated the world's largest rubber plantation in Liberia since 1926.

Question: What agreement did Liberia sign n 2006?
several multibillion-dollar concession agreements