Some people say the heart because the brain can tell it to follow. But vice versa. If we do not have a brain. The body is no longer able to work because it did not receive a command from the brain.
Some say the key because the brain can do it. , but on the other hand, If we do not have a brain The body can not function because it is not mandated by the brain.
Some say the key because it can instruct the brain do but on the other hand, if we have no brains. The body won't work because it does not take orders from the brain.