The term 'burden of ___' can be filled in with three different words: 'LLAMA', 'ONUS', and 'LOAD'. The word 'LLAMA' is a South American mammal that is often used as a pack animal. The word 'ONUS' is a noun meaning a responsibility or obligation. The word 'LOAD' is a noun meaning a heavy burden or responsibility. Therefore, the phrase 'burden of ___' can refer to the responsibility of carrying a LLAMA, the obligation of a task, or the weight of a responsibility.