Which African country is both landlocked and has the highest average elevation?

What is the world's largest landlocked country? Which landlocked country in Central Asia has the highest population? Which African landlocked country is known for its diamond mining? Which of the following European countries is landlocked? What is the name of the only landlocked country in Southeast Asia? Which landlocked country in Central Asia was once part of the Soviet Union? Which African landlocked country has a coastline on Lake Victoria? What is the name of the landlocked country in Central Europe that has a border with eight other countries?